[media] cx25821: remove unused fields, ioctls

Do some spring cleaning:
- there are no board defines with tuners, so remove bogus tuner support.
- tv standard handling has nothing to do with tuners, so keep that.
- replace the deprecated current_norm by g_std.
- querystd isn't implemented, so remove the ioctl.
- remove a bunch of unused fields in cx25821.h

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