bug#2756: move SCCS later in vc-handled-backends

[Dan Nicolaescu]

Stefan Monnier <monnier@IRO.UMontreal.CA> writes:

  > > SCCS should be moved after later in the vc-handled-backends list, after
  > > the modern VCs.
  > > Opening any file costs a call to vc-sccs-registered
  > > vc-sccs-search-project-dir (and whatever they call).
  > 
  > No matter where SCCS is in the vc-handled-backends list, those functions
  > will be run for any file that is not version controlled, which is likely
  > to be a very common case.
  >
  > > SCCS is not really popular nowadays,
  > 
  > Same thing for RCS, indeed.
  > 
  > > and that should be reflected on the vc-handled-backends list.
  > 
  > We could remove backends from the list based on the presence/absence of
  > the corresponding executable(s).

Would that be acceptable for 23.1?
If not, let's go for fixing the easier problem: moving SCCS later in the
list of backends.