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

[jloser]

Summary

There is an empty namespace std in type_traits which was originally
used when std::byte was added in
c97d8aa86650ed795bf75a7dd735ecfaef3b8f55. At some point, the bitwise operators
on std::byte got relocated but this empty namespace was left around.
Remove it.

[• Quuxplusone]

Ship it!

[Mordante]

Agreed and I would have been fine committing this without a review,

[jloser]

In D111512#3053902, @Mordante wrote:

Agreed and I would have been fine committing this without a review,

Fair enough. I still play it a bit safe at times since I'm still new to libc++ development. :)

[Mordante]

Fair point.

libcxx/include/type_traits

 
 #if _LIBCPP_STD_VER > 17
 template<bool _Const, class _Tp>
 using __maybe_const = conditional_t<_Const, const _Tp, _Tp>;
 #endif // _LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER > 14
-// std::byte
-namespace std  // purposefully not versioned
-{
-
-
-}
-#endif
-
 #endif // _LIBCPP_TYPE_TRAITS