libstdc++: Fix concept checks for std::unique_copy [PR120384]

libstdc++: Fix concept checks for std::unique_copy [PR120384]

This looks to have been wrong since r0-125454-gea89b2482f97aa which
introduced the predefined_ops.h. Since that change, the binary predicate
passed to std::__unique_copy is _Iter_comp_iter, which takes arguments
of the iterator type, not the iterator's value type.

This removes the checks from the __unique_copy overloads and moves them
into the second overload of std::unique_copy, where we have the original
binary predicate, not the adapted one from predefined_ops.h.

The third __unique_copy overload currently checks that the predicate is
callable with the input range value type and the output range value
type. This change alters that, so that we only ever check that the
predicate can be called with two arguments of the same type. That is
intentional, because calling the predicate with different types is a bug
that will be fixed in a later commit (see PR libstdc++/120386).

libstdc++-v3/ChangeLog:

	PR libstdc++/120384
	* include/bits/stl_algo.h (__unique_copy): Remove all
	_BinaryPredicateConcept concept checks.
	(unique_copy): Check _BinaryPredicateConcept in overload that
	takes a predicate.
	* testsuite/25_algorithms/unique_copy/120384.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>