[libc++] Remove redundant __invoke_constexpr functions

There are `constexpr` versions for the different `__invoke` functions, which seem to be identical other than begin `constexpr` since C++11 instead of being `constexpr` since C++20.

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D123003