[NFC][libc++][test] Removes unneeded code.

In D130295 @mumbleskates wondered why `std::strong_ordering::equal` had
special code since it's the same as `std::strong_ordering::equivalent`.

This is indeed the case so the special case can be removed.

Reviewed By: mumbleskates, #libc, avogelsgesang, ldionne

Differential Revision: https://reviews.llvm.org/D131419