[libc++] Remove __deque_base

This patch simplifies the implementation of `deque` by removing the `__deque_base` class which results in a lot less indirections and removes the need for `__base::`.

Reviewed By: ldionne, #libc

Spies: AdvenamTacet, libcxx-commits

Differential Revision: https://reviews.llvm.org/D132081