[media] tw686x-kh: remove obsolete driver

The functionality that was part of the tw686x-kh driver but not of the
mainlined tw686x driver has now been added to the tw686x driver as well.

So this driver no longer adds any functionality not available through
the mainlined driver and it can now be removed.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Krzysztof Hałasa <khalasa@piap.pl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>