drm: vc4: Remove unused vc4_fkms_cancel_page_flip function

"32a3dbeb drm/vc4: Nuke preclose hook" removed vc4_cancel_page_flip,
but vc4_fkms_cancel_page_flip was still be added to with the
fkms driver, even though it was never called.
Nuke it too.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>