[NFC] Remove non-variadic overloads of allocator_traits::construct.

Summary:
Libcxx only supports compilers with variadics. We can safely remove all "fake" variadic overloads of allocator_traits::construct.

This also provides the correct behavior if anything other than exactly one argument is supplied to allocator_traits::construct in C++03 mode.

Reviewers: ldionne, #libc!

Subscribers: dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D80067