[runtimes] Do not force -stdlib=libc++ on Apple platforms

It is pointless to specify -stdlib=libc++ on Apple platforms since
that is the default anyway.

Differential Revision: https://reviews.llvm.org/D114283

runtimes/CMakeLists.txt

@@ -81,10 +81,6 @@ set(LLVM_CMAKE_DIR ${LLVM_MAIN_SRC_DIR}/cmake/modules)
 # This variable is used by individual runtimes to locate LLVM files.
 set(LLVM_PATH ${CMAKE_CURRENT_SOURCE_DIR}/../llvm)
 
-if(APPLE)
-  set(LLVM_ENABLE_LIBCXX ON CACHE BOOL "")
-endif()
-
 include(CheckLibraryExists)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)