[gn build] Port d137e66 (#193864)

Author: nico

llvm/utils/gn/secondary/libcxx/src/BUILD.gn

@@ -131,7 +131,6 @@ cxx_sources = [
   "functional.cpp",
   "future.cpp",
   "hash.cpp",
-  "include/apple_availability.h",
   "include/atomic_support.h",
   "include/config_elast.h",
   "include/from_chars_floating_point.h",