V4L/DVB (3525): Kconfig: remove VIDEO_DECODER

- removed VIDEO_DECODER Kconfig menu item.
  VIDEO_CX25840, VIDEO_SAA711X and VIDEO_SAA7127 now
  each have their own menu items.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>