[media] media: remove media entity .parent field

Now that the struct media_entity .parent field is unused, it can be
safely removed. Since all the previous users were converted to use
the .mdev field from the embedded struct media_gobj instead.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>