V4L/DVB: Documentation: update now that the vtx/videotext API has been removed

Remove all references to /dev/vtx in the documentation, except for
some historical comments in dev-teletext.xml.

Documentation/devices.txt is not updated, this will go through Alan Cox
who maintains this file.

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