[libc++][NFC] Remove the old PSTL headers

We don't actually use these headers, so we might as well remove them.

Reviewed By: ldionne, #libc

Spies: arichardson, EricWF, sstefan1, jplehr, wangpc, mgrang, libcxx-commits, miyuki

Differential Revision: https://reviews.llvm.org/D157558