[media] dvb: don't keep support for undocumented features

There are two DVB demux callbacks and ioctls that aren't documented
and aren't used at all by the DVB core or by any DVB driver upstream.

Let's comment out the code for those two ioctls and remove on some
future version.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>