[libc++] Don't open-close namespace std in __config (#72695)

This doesn't seem necessary and it is just kind of weird to do that in
__config, so remove it.

Author: ldionne

libcxx/include/__config

@@ -836,8 +836,6 @@ typedef __char32_t char32_t;
 #  define _LIBCPP_END_NAMESPACE_STD }}
 #  define _VSTD std
 
-_LIBCPP_BEGIN_NAMESPACE_STD _LIBCPP_END_NAMESPACE_STD
-
 #  if _LIBCPP_STD_VER >= 17
 #    define _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM                                                                         \
        _LIBCPP_BEGIN_NAMESPACE_STD inline namespace __fs { namespace filesystem {