[media] cx25821: remove video output support

The video output functionality never worked for this driver. Now remove the
creation of the output video nodes as well to prevent users from thinking
that video output is available, when it isn't.

To correctly implement this the video output should use vb2 as well, and
that requires rewriting the output DMA setup. But without hardware to test
I am not able to do that.

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