[libc++] Remove empty namespace std in type_traits. NFCI.

There is an empty `namespace std` in `type_traits` which was originally
used when `std::byte` was added in
c97d8aa866. At some point, the bitwise operators
on `std::byte` got relocated but this empty namespace was left around.
Remove it.

Reviewed By: Quuxplusone, Mordante, #libc

Differential Revision: https://reviews.llvm.org/D111512

libcxx/include/type_traits

@@ -4175,13 +4175,4 @@ using __maybe_const = conditional_t<_Const, const _Tp, _Tp>;
 
 _LIBCPP_END_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER > 14
-// std::byte
-namespace std  // purposefully not versioned
-{
-
-
-}
-#endif
-
 #endif // _LIBCPP_TYPE_TRAITS