[libc++] Remove std::function in C++03

We've said that we'll remove `std::function` from C++03 in LLVM 16, so we might as well do it now before we forget.

Reviewed By: ldionne, #libc, Mordante

Spies: jloser, Mordante, libcxx-commits

Differential Revision: https://reviews.llvm.org/D135868