libstdc++: Remove incorrect copyright notice from header

This file has the SGI copyright notice, but contains no code from
the SGI STL. It was entirely written by me in 2019, originally as part
of the <memory> header. When I extracted it into a new header I
accidentally copied across the SGI copyright, but that only applies to
some much older parts of <memory>.

libstdc++-v3/ChangeLog:

	* include/bits/uses_allocator_args.h: Remove incorrect copyright
	notice.