[DotNet] BuildAnalysis does not show deleted files in the PR

The `Code Check` script is using `git diff` command to check if the code is up-to-date, therefore it might not be able to catch the "untracked" files in git: https://github.com/Azure/azure-sdk-for-net/blob/dfcc1226d2de34048723977f499f40fb4abe34eb/eng/scripts/CodeChecks.ps1#L165
Therefore if we just remove some files in a PR, the regeneration will recreate those removed files. Since these files are untracked and not got catched in `git diff`, the CI will pass, which is a false negative.