[libc++][NFC] Remove __has_keyword

__has_keyword is almost not used anymore. There are only two cases. One can be replaced by __has_builtin and the other seems entirely redundant, so we can remove the definition.

Reviewed By: #libc, Mordante

Spies: Mordante, libcxx-commits

Differential Revision: https://reviews.llvm.org/D158215