[libc++][spaceship] P1614R2: Removed `operator!=` from `weekday`s

Implements parts of P1614R2. Removed `operator!=` from:
- `weekday`
- `weekday_indexed`
- `weekday_last`
- `month_weekday`
- `month_weekday_last`
- `year_month_weekday`
- `year_month_weekday_last`

Note these operators were added and removed in C++20.

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D152699