moving SCCS later in vc-handled-backends

[Stephen J. Turnbull]

Dan Nicolaescu writes:
 > 
 > When opening any version controlled file that is under one of the more
 > modern VCs, SCCS is tried first.
 > Given that it's not a popular system, this is pure overhead,
 > so it would be better move it at the end of vc-handled-backends.
 > Any objection to this change:

Yes.  This change is backward incompatible and buggy; the rationale
for SCCS being in the "early" group still holds as far as I know,
unless SCCS has improved its support for whole-tree commits recently.

Just deprecate it entirely, by removing it from the list.  Maybe
document that 'SCCS is an allowed element, and it should come early if
it's used.