[media] media: drop call to v4l2_device_unregister_subdev()

These drivers are moved to support asynchronous probing,
v4l2_async_unregister_subdev() unregisters the subdev so
there isn't a need to explicitly call v4l2_device_unregister_subdev().

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>