[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
c97d8aa8. 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