libstdc++: Reuse std::__assign_one in <bits/ranges_algobase.h>

Use std::__assign_one instead of ranges::__assign_one. Adjust the uses,
because std::__assign_one has the arguments in the opposite order (the
same order as an assignment expression).

libstdc++-v3/ChangeLog:

	* include/bits/ranges_algobase.h (ranges::__assign_one): Remove.
	(__copy_or_move, __copy_or_move_backward): Use std::__assign_one
	instead of ranges::__assign_one.

Reviewed-by: Patrick Palka <ppalka@redhat.com>