Remove VC rollback method.

* vc/vc.el, vc/vc-hg.el, vc/vc-git.el, vc/vc-hooks.el,
vc/vc-mtn.el, vc/vc-rcs.el, vc/vc-sccs.el, vc/vc-src.el: rrollback
method removed, to be replaced in the future by uncommit.