[libc++] Removes __cpp_lib_monadic_optional.

P0798R8 "Monadic operations for std::optional" has been implemented, so
this LWG issue can be adopted.

During review it was discovered another paper bumped the macro. The
part affecting optional of this paper is done, the variant isn't. The
status page is updated to reflect the current state.

Implements
- LWG 3621 Remove feature-test macro __cpp_lib_monadic_optional

Updates status of
- P2231R1 Missing constexpr in std::optional and std::variant

Reviewed By: #libc, philnik, ldionne

Differential Revision: https://reviews.llvm.org/D125813