[libc++][NFC] Remove unused include in <compare>.

`<type_traits>` was included in the first iteration of `<compare>` when
it was created as a monolithic header, then never removed. Removing it
now is a beneficial no-op since it is not guaranteed by the standard
and is already included by all of its subheaders.

Reviewed By: cjdb, #libc, Quuxplusone

Differential Revision: https://reviews.llvm.org/D107801