build: swig and contrib/rapidjson

we set swig dir before find swig package, the var `SWIG_DIR` will be set by find cmake, no need to set here, and we have already set .deps/usr/ to be find path.
- [ ] may failed on hybridsql, but it can be find without swig_dir in centos6?

```
-- Adding contrib module rapidjson (configuring with rapidjson)
CMake Deprecation Warning at contrib/rapidjson/CMakeLists.txt:1 (CMAKE_MINIMUM_REQUIRED):
Compatibility with CMake < 2.8.12 will be removed from a future version of
-- why trigger again?
-- CMAKE_PREFIX_PATH: /root/OpenMLDB/.deps/usr
-- CMAKE_BUILD_TYPE: RelWithDebInfo
-- MAC_TABLET_ENABLE: ON
-- hw test /root/OpenMLDB/.deps/usr/share/swig/4.0.1
-- Found Protobuf Libraries: /root/OpenMLDB/.deps/usr/lib/libprotobuf.a;-lpthread
-- Adding contrib module simdjson (configuring with simdjson)
CMake Error at contrib/simdjson/CMakeLists.txt:1 (add_library):
add_library cannot create target "_simdjson" because another target with
the same name already exists. The existing target is a static library
created in source directory "/root/OpenMLDB/contrib/simdjson". See
documentation for policy CMP0002 for more details.

-- Adding contrib module rapidjson (configuring with rapidjson)
CMake Deprecation Warning at contrib/rapidjson/CMakeLists.txt:1 (CMAKE_MINIMUM_REQUIRED):
Compatibility with CMake < 2.8.12 will be removed from a future version of
CMake.

Update the VERSION argument value or use a ... suffix to tell
CMake that the project does not need compatibility with older versions.

CMake Error at contrib/rapidjson/doc/CMakeLists.txt:21 (add_custom_target):
add_custom_target cannot create target "doc" because another target with
the same name already exists. The existing target is a custom target
created in source directory "/root/OpenMLDB/contrib/rapidjson/doc". See
documentation for policy CMP0002 for more details.
```
contrib projects are not robust, rerun cmake may got dup targets. The cmake log shows it re-run after rapidjson module, so I make rapidjson thinner, don't build test or doc in it. Then cmake will be fine.