[media] em28xx: remove sliced VBI support

The sliced VBI support in the tvp5150 is completely broken. And there is no
support for the saa7115 sliced VBI implementation in the em28xx driver. So
we remove the sliced VBI support completely.
It should be possible to get it to work with the tvp5150, but that will
require someone to really dig into that driver.

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