V4L/DVB (10329): af9015: remove dual_mode module param

Remove dual_mode module	param. Possible 2nd FE seems not to be buggy
any more and therefore can be enabled as default.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>