[libcxx] Stop recommending setting LIBCXX_HAS_WIN32_THREAD_API in the MinGW builds

Since a8d15a9266 / D110975, this is
the default, even if winpthread headers are available, so we don't
need to cargo cult setting this option in all builds.

Differential Revision: https://reviews.llvm.org/D122717

libcxx/cmake/caches/MinGW.cmake

@@ -1,5 +1,4 @@
 set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
-set(LIBCXX_HAS_WIN32_THREAD_API ON CACHE BOOL "")
 
 set(LIBCXX_CXX_ABI libcxxabi CACHE STRING "")
 set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")

libcxx/docs/BuildingLibcxx.rst

@@ -159,7 +159,6 @@ e.g. the ``mingw-w64-x86_64-clang`` package), together with CMake and ninja.
           -DCMAKE_C_COMPILER=clang                                                    \
           -DCMAKE_CXX_COMPILER=clang++                                                \
           -DLLVM_ENABLE_RUNTIMES=libcxx                                               \
-          -DLIBCXX_HAS_WIN32_THREAD_API=ON                                            \
           -DLIBCXX_CXX_ABI=libstdc++                                                  \
           -DLIBCXX_TARGET_INFO="libcxx.test.target_info.MingwLocalTI"
   > ninja -C build cxx