libstdc++: Undeprecate std::pmr::polymorphic_allocator::destroy (P2875R4)

libstdc++: Undeprecate std::pmr::polymorphic_allocator::destroy (P2875R4)

This member function was previously deprecated, but that was reverted by
P2875R4, approved earlier this year in Tokyo. Since it's not going to be
deprecated in C++26, and so presumably not removed, there is no point in
giving deprecated warnings for C++23 mode.

libstdc++-v3/ChangeLog:

	* include/bits/memory_resource.h (polymorphic_allocator::destroy):
	Remove deprecated attribute.