[libcxx] [ci] Don't disable libc++experimental in mingw builds

Since dfa88927ae, the static
libc++experimental should work in mingw dll builds. (It probably worked
all along in static mingw builds.)

Differential Revision: https://reviews.llvm.org/D129270

libcxx/cmake/caches/MinGW.cmake

@@ -1,5 +1,3 @@
-set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
-
 set(LIBCXX_CXX_ABI libcxxabi CACHE STRING "")
 set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")