[media] v4l: omap4iss: Remove duplicate video_is_registered() check

The video_unregister_device() function checks if the video device is
registered before proceeding, remote the duplicate check from the
driver.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>