[media] v4l2: remove deprecated current_norm support completely

The use of current_norm to keep track of the current standard has been
deprecated for quite some time. Now that all drivers that were using it
have been converted to use g_std we can drop it from the core.
It was a bad idea to introduce this at the time: since it is a per-device
node field it didn't work for drivers that create multiple nodes, all sharing
the same tuner (e.g. video and vbi nodes, or a raw video node and a compressed
video node). In addition it was very surprising behavior that g_std was
implemented in the core. Often drivers implemented both g_std and current_norm,
because they didn't understand how it should be used.
Since the benefits were very limited (if they were there at all), it is better
to just drop it and require that drivers just implement g_std.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>