[libc++][z/OS] __cxx03 subdir was added by mistake (llvm#122763)

The header <features.h> is a system header. It's not part of the headers
in __cxx03.

Author: perry-ca

libcxx/include/__cxx03/__config

@@ -230,7 +230,7 @@ _LIBCPP_HARDENING_MODE_DEBUG
 #  endif
 
 #  if defined(__MVS__)
-#    include <__cxx03/features.h> // for __NATIVE_ASCII_F
+#    include <features.h> // for __NATIVE_ASCII_F
 #  endif
 
 #  if defined(_WIN32)