build-libcxx: Don't explicitly disable libc++ std::filesystem

It's currently disabled by default for windows targets; rely on this.
If it's enabled automatically upstream later, take it into use.

build-libcxx.sh

@@ -162,7 +162,6 @@ build_all() {
             -DLIBCXX_SUPPORTS_STD_EQ_CXX11_FLAG=TRUE \
             -DLIBCXX_HAVE_CXX_ATOMICS_WITHOUT_LIB=TRUE \
             -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=OFF \
-            -DLIBCXX_ENABLE_FILESYSTEM=OFF \
             -DLIBCXX_ENABLE_STATIC_ABI_LIBRARY=TRUE \
             -DLIBCXX_ENABLE_NEW_DELETE_DEFINITIONS=ON \
             -DLIBCXX_CXX_ABI=libcxxabi \