Delete struct artdata archive_head

Delete struct artdata archive_head

This element is unused.  Ideally we'd be moving archive_head and
other archive specific fields from struct bfd to here, but that's a
much larger change than this little bit of cleanup.

	* libbfd-in.h (struct artdata): Delete archive_head.
	* libbfd.h: Regenerate.
	* archive.c,
	* coff-rs6000.c,
	* coff64-rs6000.c: Delete comments mentioning artdata archive_head.