fix interactive spec for kill-region

[T.V. Raman]

Saw an unexplained bytecomp warning in the emacspeak build and
traced it down the following:

The definition of kill-region appears not to match its
interactive spec --- the  optional third arg  does not appear in
the interactive spec.
See
(defun kill-region (beg end &optional yank-handler)
vs
(interactive (list (point) (mark)))

-- 
Best Regards,
--raman