[libc++][NFC] Simplify duration comparisons a bit (#201788)

The comparisons have been delegated to a class which has been
specialized for the equality case. This has likely been done to avoid
`common_type` if possible. However, `common_type` got a lot cheaper, to
the point where the classes likely do more harm than good.

GitOrigin-RevId: 013b33583b632f4c680db82f42223e3684da8cb2