Non-existent VS kits aren't removed

### Brief Issue Summary

When VS no longer exists on the system (i.e. uninstalling them), I expected the cached kits to be removed the next time I scan for kits, but that's **not** the case.

### Expected:

1. Remove VS (I uninstalled it, not aware of other ways to make it "invisible").
2. Scan kits and check the kits again, the previous VS toolkits will still be there.

### CMake Tools Log

```
[variant] Loaded new set of variants
[kit] Successfully loaded 13 kits from C:\Users\Sam Huang\AppData\Local\CMakeTools\cmake-tools-kits.json
[main] Configuring folder: mpp
[proc] Executing command: "C:\Program Files\CMake\bin\cmake.EXE" --no-warn-unused-cli -DMPP_BUILD_TESTS=TRUE -DCMAKE_EXPORT_COMPILE_COMMANDS:BOOL=TRUE -DCMAKE_BUILD_TYPE:STRING=Debug "-Hc:/Users/Sam Huang/Projects/mpp" "-Bc:/Users/Sam Huang/Projects/mpp/build" -G Ninja
[cmake] Not searching for unused variables given on the command line.
[cmake] -- Configuring done
[cmake] -- Generating done
[cmake] -- Build files have been written to: C:/Users/Sam Huang/Projects/mpp/build
[proc] Executing command: C:\msys64\mingw64\bin\gcc.exe -v
[proc] Executing command: C:\msys64\mingw64\bin\x86_64-w64-mingw32-gcc-10.3.0.exe -v
[proc] Executing command: C:\msys64\mingw64\bin\x86_64-w64-mingw32-gcc.exe -v
[kit] Found Kit: GCC 10.3.0 x86_64-w64-mingw32
[kit] Found Kit: GCC 10.3.0 x86_64-w64-mingw32
[kit] Found Kit: GCC 10.3.0 x86_64-w64-mingw32
[kit] Successfully loaded 13 kits from C:\Users\Sam Huang\AppData\Local\CMakeTools\cmake-tools-kits.json
```

### Developer Tools Log

```
workbench.desktop.main.js:sourcemap:62 [Extension Host] (node:12596) [DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.(Use `Code --trace-deprecation ...` to show where the warning was created)
m @ workbench.desktop.main.js:sourcemap:62
workbench.desktop.main.js:sourcemap:2844 [twxs.cmake]: Cannot register 'cmake.cmakePath'. This property is already registered.
_logMessageInConsole @ workbench.desktop.main.js:sourcemap:2844
marketplace.visualstudio.com/_apis/public/gallery/extensionquery:1 Failed to load resource: net::ERR_CONNECTION_RESET
workbench.desktop.main.js:sourcemap:62 [Extension Host] activating extension
workbench.desktop.main.js:sourcemap:62 [Extension Host] starting language server
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:30.255Z [debug] [main] Safe constructing new CMakeTools instance
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:30.256Z [debug] [variant] Constructing VariantManager
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:30.258Z [debug] [main] Constructing new CMakeTools instance
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:30.258Z [debug] [main] Starting CMakeTools second-phase init
workbench.desktop.main.js:sourcemap:62 [Extension Host] strict mode: missing type "object" for keyword "required" at "#/patternProperties/%5E%5Cw%2B%24/properties/choices/patternProperties/%5E%5Cw%2B%24" (strictTypes)
m @ workbench.desktop.main.js:sourcemap:62
workbench.desktop.main.js:sourcemap:62 [Extension Host] strict mode: missing type "object" for keyword "properties" at "#/patternProperties/%5E%5Cw%2B%24/properties/choices/patternProperties/%5E%5Cw%2B%24" (strictTypes)
m @ workbench.desktop.main.js:sourcemap:62
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:31.023Z [info] [variant] Loaded new set of variants
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:31.024Z [debug] [main] CMakeTools instance initialization complete.
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:31.029Z [debug] [kit] Reading kits file C:\Users\Sam Huang\AppData\Local\CMakeTools\cmake-tools-kits.json
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:31.146Z [info] [kit] Successfully loaded 13 kits from C:\Users\Sam Huang\AppData\Local\CMakeTools\cmake-tools-kits.json
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:31.166Z [debug] [kit] Not reading non-existent kits file: c:\Users\Sam Huang\Projects\mpp\.vscode\cmake-kits.json
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:31.192Z [debug] [main] Injecting new Kit into CMake driver
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:31.240Z [debug] [extension] Configuring workspace on open file:///c%3A/Users/Sam%20Huang/Projects/mpp
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:31.241Z [info] [main] Configuring folder: mpp
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:31.243Z [debug] [main] Saving open files before configure/build
workbench.desktop.main.js:sourcemap:62 [Extension Host] updateCustomBrowseConfiguration
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.315Z [debug] [main] Starting new CMake driver
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.316Z [debug] [main] Starting CMake driver
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.316Z [debug] [main] Using generator from user configuration: Ninja
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.317Z [debug] [cmakefileapi-driver] Creating instance of CMakeFileApiDriver
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.319Z [debug] [driver] Initializating base driver using kit
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.320Z [debug] [driver] CMakeDriver Kit set to __unspec__
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.326Z [debug] [driver] Run _refreshExpansions
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.327Z [debug] [driver] Run _refreshExpansions cb
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.498Z [debug] [cache] Reading CMake cache file c:/Users/Sam Huang/Projects/mpp/build/CMakeCache.txt
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.601Z [debug] [cache] Parsing CMake cache string
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.633Z [debug] [cache] Reading CMake cache file c:/Users/Sam Huang/Projects/mpp/build/CMakeCache.txt
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.642Z [debug] [cache] Parsing CMake cache string
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.670Z [debug] [cmakefileapi-parser] Read reply folder: c:\Users\Sam Huang\Projects\mpp\build\.cmake\api\v1\reply
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.695Z [debug] [cmakefileapi-parser] Found index files: ["cache-v2-b0f993b30340d9846b0c.json","codemodel-v2-4dcaf3e51af1c20e1a75.json","index-2021-07-22T21-42-26-0498.json","target-algos_test-Debug-7a971885e6a50ae4045a.json","target-ariths_test-Debug-faec69834ae1502d5525.json","target-assign_test-Debug-9bbc15e8443ff96bbdb0.json","target-customize_test-Debug-cc0026e4c0c252a304f3.json","target-init_test-Debug-c962eabe358effcd4482.json","target-iter_test-Debug-6aabd778e94d5321bf66.json","target-mem_fns_test-Debug-9e42d405bd5e6ff39d90.json","target-tests-Debug-dd48eb3353d3f9f57292.json","target-utils_test-Debug-e5e7f4197775b08fa08a.json","toolchains-v1-0b60f7fb1411fae0baf8.json"]
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.750Z [debug] [driver] Setting new variant Debug
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.751Z [debug] [driver] Run _refreshExpansions
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:32.751Z [debug] [driver] Run _refreshExpansions cb
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.182Z [debug] [driver] Run _refreshExpansions
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.182Z [debug] [driver] Run _refreshExpansions cb
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.191Z [debug] [driver] Start configure
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.193Z [debug] [driver] Runnnig pre-configure checks and steps
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.201Z [debug] [driver] Run _refreshExpansions
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.201Z [debug] [driver] Run _refreshExpansions cb
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.217Z [debug] [cmakefileapi-driver] Configuring using kit
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.217Z [debug] [cmakefileapi-driver] Invoking CMake C:\Program Files\CMake\bin\cmake.EXE with arguments ["--no-warn-unused-cli","-DMPP_BUILD_TESTS=TRUE","-DCMAKE_EXPORT_COMPILE_COMMANDS:BOOL=TRUE","-DCMAKE_BUILD_TYPE:STRING=Debug","-Hc:/Users/Sam Huang/Projects/mpp","-Bc:/Users/Sam Huang/Projects/mpp/build","-G","Ninja"]
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.227Z [info] [proc] Executing command: "C:\Program Files\CMake\bin\cmake.EXE" --no-warn-unused-cli -DMPP_BUILD_TESTS=TRUE -DCMAKE_EXPORT_COMPILE_COMMANDS:BOOL=TRUE -DCMAKE_BUILD_TYPE:STRING=Debug "-Hc:/Users/Sam Huang/Projects/mpp" "-Bc:/Users/Sam Huang/Projects/mpp/build" -G Ninja
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.457Z [info] [cmake] Not searching for unused variables given on the command line.
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.632Z [info] [cmake] -- Configuring done
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.743Z [info] [cmake] -- Generating done
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.896Z [info] [cmake] -- Build files have been written to: C:/Users/Sam Huang/Projects/mpp/build
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.903Z [debug] [cmakefileapi-parser] Read reply folder: c:\Users\Sam Huang\Projects\mpp\build\.cmake\api\v1\reply
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:33.907Z [debug] [cmakefileapi-parser] Found index files: ["cache-v2-b0f993b30340d9846b0c.json","codemodel-v2-4dcaf3e51af1c20e1a75.json","index-2021-07-22T21-44-33-0894.json","target-algos_test-Debug-7a971885e6a50ae4045a.json","target-ariths_test-Debug-faec69834ae1502d5525.json","target-assign_test-Debug-9bbc15e8443ff96bbdb0.json","target-customize_test-Debug-cc0026e4c0c252a304f3.json","target-init_test-Debug-c962eabe358effcd4482.json","target-iter_test-Debug-6aabd778e94d5321bf66.json","target-mem_fns_test-Debug-9e42d405bd5e6ff39d90.json","target-tests-Debug-dd48eb3353d3f9f57292.json","target-utils_test-Debug-e5e7f4197775b08fa08a.json","toolchains-v1-0b60f7fb1411fae0baf8.json"]
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:34.073Z [debug] [cache] Reading CMake cache file c:/Users/Sam Huang/Projects/mpp/build/CMakeCache.txt
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:34.082Z [debug] [cache] Parsing CMake cache string
workbench.desktop.main.js:sourcemap:62 [Extension Host] updateCustomBrowseConfiguration
workbench.desktop.main.js:sourcemap:62 [Extension Host] provideCustomConfiguration
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:34.534Z [debug] [cache] Reading CMake cache file c:/Users/Sam Huang/Projects/mpp/build/CMakeCache.txt
workbench.desktop.main.js:sourcemap:62 [Extension Host] [CMakeTools] 2021-07-22T21:44:34.536Z [debug] [cache] Parsing CMake cache string
workbench.desktop.main.js:sourcemap:62 [Extension Host] [vscode-icons] v11.5.0 activated!
workbench.desktop.main.js:sourcemap:62 [Extension Host] command:editor.action.addCommentLine
```

### Platform and Versions

- **Operating System**: Windows
- **CMake Version**: 3.20.5
- **VSCode Version**: 1.58.2
- **CMake Tools Extension Version**: 1.7.3
- **Compiler/Toolchain**: Visual C++ 2019 Preview

### Other Notes/Information

Start by tracing the kit scan that loads cmake-tools-kits.json and compare the discovered kits with the cached entries reported in the issue log. Reproduce the uninstall-and-rescan sequence, then verify that kits whose Visual Studio installation no longer exists are removed while valid kits remain.

Written by the indexing model from the issue text.