Revert "[libc++] Implement P2505R5(Monadic operations for std::expected)."

This reverts commit ebc111b0.

Sorry, I forgot to append Phabricator reversion when land D140911, I try to revert this change and reland.

Reviewed By: #libc, ldionne, EricWF

Differential Revision: https://reviews.llvm.org/D150793