[libcxx] removes operator!= and globally guards against no spaceship operator

* `operator!=` isn't in the spec
* `<compare>` is designed to work with `operator<=>` so it doesn't
  really make sense to have `operator<=>`-less friendly sections.

Depends on D100283.

Differential Revision: https://reviews.llvm.org/D100342

NOKEYCHECK=True
GitOrigin-RevId: 20506fb1f361e41012506c6c252fd690541fc708
5 files changed