V4L/DVB (3692): Keep experimental SLICED_VBI defines under an #if 0

The sliced VBI defines added in videodev2.h are removed since requires
more discussion.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>