Remove never-used rev argument from VC's backend checkin methods.

Alters vc/vc-arch.el, vc/vc-bzr.el, vc/vc-cvs.el, vc/vc-dav.el,
vc/vc-git.el, vc/vc-hg.el, vc/vc-mtn.el, vc/vc-rcs.el,
vc/vc-sccs.el, vc/vc-svn.el, vc/vc.el.

Only the RCS, SCCS, and CVS back ends tried to do anything with it,
and that code was never exercised. Chiseling away the cruft of
decades...