Renove duplicate code.
API simplification: remove vc-workfile-unchanged-p from pubic methods.

* vc/vc.el, vc-hooks.el, and all backends: API simplification;
vc-workfile-unchanged-p is no longer a public method (but the RCS and
SCCS back ends retain it as a private method used in state
computation). This method was redundant with vc-state and usually
implemented as a trivial call to same. Fixes the failure mode
described in bug#694.