[libc++][NFC] Mark LWG2731 as complete

Mark LWG2731 as complete. The type alias `mutex_type` is only provided if
`scoped_lock` is given one mutex type and it has been implemented that
way since the beginning of Clang 5 it seems. There already are tests for
verifying existence (and lack thereof) for `mutex_type` type alias
depending on the number of mutex types, so there is nothing to
do for this LWG issue.

Reviewed By: Quuxplusone, Mordante, #libc

Differential Revision: https://reviews.llvm.org/D112462