media: v4l: uAPI doc: Simplify NATIVE_SIZE selection target documentation

The NATIVE_SIZE target is documented for mem2mem devices but no driver has
ever apparently used it. It may be never needed; remove it for now.

Suggested-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>