[media] dvb: get rid of fe_ioctl_override callback

This callback were meant to allow overriding a FE callback, before its
call, but it is not really needed, as the callback can be intercepted
after tuner attachment.

Worse than that, only DVBv3 calls are intercepted this way, so a DVBv5
application will produce different effects than a DVBv3 one.

So, get rid of it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Acked-by: Antti Palosaari <crope@iki.fi>
Reviewed-by: Antti Palosaari <crope@iki.fi>