media: omap3isp: Drop custom .unsubscribe_event() handler

The ccdc_unsubscribe_event() and omap3isp_stat_unsubscribe_event()
functions simply call v4l2_event_unsubscribe(), forwarding their
arguments. Replace them with the v4l2_event_subdev_unsubscribe() helper
that performs exactly the same operation.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>