PR libstdc++/86537 remove less<shared_ptr<T>> partial specialization

PR libstdc++/86537 remove less<shared_ptr<T>> partial specialization

The standard doesn't specify this partial specialization (it was
required after the changes in N2637 but then should have been removed
following LWG 1262). Its presence is observable because it causes
different results when operator< has been overloaded for a shared_ptr
specialization.

	PR libstdc++/86537
	* include/bits/shared_ptr.h (less<shared_ptr<_Tp>>): Remove
	non-standard partial specialization.
	* include/bits/shared_ptr_base.h (_Sp_less): Remove class definition.
	(less<__shared_ptr<_Tp, _Lp>): Remove partial specialization.
	* testsuite/20_util/shared_ptr/comparison/86537.cc: New test.

From-SVN: r262739