[libc++] Remove <experimental/{algorithm,functional}>

The contents of these files have been implemented for at least two releases, so let's remove them according to our policy (https://libcxx.llvm.org/DesignDocs/ExperimentalFeatures.html#id4).

Reviewed By: Mordante, #libc

Spies: arichardson, libcxx-commits

Differential Revision: https://reviews.llvm.org/D144475