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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@334069 91177308-0d34-0410-b5e6-96231b3b80d8

include/__functional_base

@@ -646,16 +646,6 @@ void __user_alloc_construct_impl (integral_constant<int, 2>, _Tp *__storage, con
     new (__storage) _Tp (_VSTD::forward<_Args>(__args)..., __a);
 }
 
-// FIXME: Theis should have a version which takes a non-const alloc.
-template <class _Tp, class _Allocator, class... _Args>
-inline _LIBCPP_INLINE_VISIBILITY
-void __user_alloc_construct (_Tp *__storage, const _Allocator &__a, _Args &&... __args)
-{ 
-    __user_alloc_construct_impl( 
-             __uses_alloc_ctor<_Tp, _Allocator>(), 
-             __storage, __a, _VSTD::forward<_Args>(__args)...
-        );
-}
 #endif  // _LIBCPP_CXX03_LANG
 
 _LIBCPP_END_NAMESPACE_STD