Remove editable argument from VC's backend checkout methods.

Alters 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.

where this matters (which is only in SCCS and RCS) files are now always
checked out editable. This may actually have been dynamically true
already - it looks like the vc-next-action code evolved past visiting
the other case. Tested with RCS.