[media] adv7180: Remove duplicate unregister call

This driver moved over to v4l2_async_unregister_subdev()
but still retained a call to v4l2_unregister_subdev(). Remove.

Signed-off-by: Ian Molton <ian.molton@codethink.co.uk>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>