[libc++] [P0646] Add feature-test macro for __cpp_lib_list_remove_return_type.

Summary: The return type modification has already been implemented in rL364840 and rL365290.

Reviewers: ldionne, mclow.lists, EricWF, #libc!

Reviewed By: ldionne

Subscribers: christof, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D70275