[media] cx231xx: remove btcx_riscmem reference

Remove this comment block, it's unused. This removes the btcx_riscmem
reference as well, since that will no longer be available to other
drivers except bttv.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>