[libcxx][atomic] Remove workaround for PR31864

I believe the origin issue in PR31864 has been addressed by https://reviews.llvm.org/D59566.

As discussed in https://github.com/llvm/llvm-project/issues/53840, `ATOMIC_LLONG_LOCK_FREE == 2` sometimes is not consistent with `std::atomic<long long>::is_always_lock_free`, since the macro takes `long long`'s ABI alignment into account. https://reviews.llvm.org/D28213 proposed we should not rely on ABI alignment of types, thus we have consistent `ATOMIC_LLONG_LOCK_FREE` and `std::atomic<long long>::is_always_lock_free` on x86's old cpu. Currently, I plan to move on to remove the workaround which should have been addressed and don't want to break current tests.

Reviewed By: #libc, ldionne, Quuxplusone

Differential Revision: https://reviews.llvm.org/D119931