media: staging: atomisp: Drop custom .unsubscribe_event() handler

The isp_subdev_unsubscribe_event() function simply calls
v4l2_event_unsubscribe(), forwarding its arguments. Replace it 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>