Minor cleanups in ediff-*.el

Minor cleanups in ediff-*.el

* lisp/vc/ediff-init.el (ediff-window-visible-p):
(ediff-nonempty-string-p): Simplify.
(stipple-pixmap): Don't try XEmacs specific(?) variable.
(ediff-frame-unsplittable-p): Make obsolete.  Update callers.
(ediff-has-gutter-support-p): Redefine as obsolete function alias for
'ignore'.
(ediff-mouse-event-p): Redefine as obsolete function alias for
'mouse-event-p'.  Update callers.
* lisp/vc/ediff-util.el (ediff-submit-report): Simplify.
(ediff-filename-magic-p): Make obsolete.  Update callers.
(ediff-activate-mark): Make obsolete.  Update callers.
* lisp/vc/ediff-help.el:
* lisp/vc/ediff-hook.el:
* lisp/vc/ediff-util.el:
* lisp/vc/ediff-wind.el: Remove superfluous defvars.