V4L/DVB (3318e): DVB: remove the at76c651/tda80xx frontends

The at76c651 and tda80xx frontends are currently completely unused, IOW
their only effect is making the kernel larger for people accitentially
enabling them.

The current in-kernel drivers differ from the drivers at cvs.tuxbox.org,
and re-adding them when parts of the dbox2 project get merged should be
trivial.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>