media: v4l2-common: get rid of v4l2_routing dead struct

This struct is not used anymore. Get rid of it and update
the documentation about what should still be converted.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>