[libcxx] Stop recommending setting LIBCXX_HAS_WIN32_THREAD_API in the MinGW builds

[mstorsjo]

Summary

Since a8d15a926689c126c4d316788786e0160cfc1d5d / 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.

[mstorsjo]

Ping @ldionne

libcxx/cmake/caches/MinGW.cmake

 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 "")
 
 set(LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
 set(LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
 
 set(LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
 set(LIBCXXABI_USE_COMPILER_RT ON CACHE BOOL "")
 set(LIBUNWIND_USE_COMPILER_RT ON CACHE BOOL "")
 
 set(LIBCXX_TARGET_INFO "libcxx.test.target_info.MingwLocalTI" CACHE STRING "")
 
 # Without this flag, 'long double' (which is 80 bit on x86 mingw, but
 # 64 bit in MSVC) isn't handled correctly in printf.
 set(LIBCXX_EXTRA_SITE_DEFINES "__USE_MINGW_ANSI_STDIO=1" CACHE STRING "")

libcxx/docs/BuildingLibcxx.rst

 e.g. the ``mingw-w64-x86_64-clang`` package), together with CMake and ninja.
 
 .. code-block:: bash
 
   > cmake -G Ninja -S runtimes -B build                                               \
           -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
   > cp /mingw64/bin/{libstdc++-6,libgcc_s_seh-1,libwinpthread-1}.dll lib
   > ninja -C build check-cxx
 
 As this build configuration ends up depending on a couple other DLLs that
 aren't available in path while running tests, copy them into the same