Remove unused code from __functional_base. NFC.

Patch from Arthur O'Dwyer.

`__user_alloc_construct_impl` is used by <experimental/memory_resource>, but
this `__user_alloc_construct` is never used.

Also, `<experimental/memory_resource>` doesn't need a full definition of
`std::tuple`; just the forward declaration in `<__tuple>` will suffice.

Reviewed as https://reviews.llvm.org/D46806

llvm-svn: 334069