[PR middle-end/113525] Drop obsolete options from documentation

The sibling and unshare passes were dropped as distinct passes 10+ years ago.
Docs weren't ever updated.  This just removes them; given their age I don't
think we need to keep them around any longer.

	PR middle-end/113525

gcc/
	* doc/invoke.texi (dump-rtl-sibling): Drop documentation for pass
	removed long ago.
	(dump-rtl-unshare): Likewise.