[media] v4l2-async: remove unneeded .registered_async callback

The v4l2_subdev_core_ops .registered_async callback was added to notify
a subdev when its entity has been registered with the media device, to
allow for example to modify the media graph (i.e: adding entities/links).

But that's not needed since there is already a .registered callback in
struct v4l2_subdev_internal_ops that's called after the entity has been
registered with the media device in v4l2_device_register_subdev().

Suggested-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>