[libc++] Remove unused __functional includes

Reviewed By: ldionne, #libc

Spies: arichardson, smeenai, libcxx-commits, arphaman

Differential Revision: https://reviews.llvm.org/D126098

NOKEYCHECK=True
GitOrigin-RevId: 34f73804ed60e90276e2303398162c2fc05f47ec
32 files changed