doc-rst: Remove the media docbook

Now that all media documentation was converted to Sphinx, we
should get rid of the old DocBook one, as we don't want people
to submit patches against the old stuff.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>