[media] af9033: wrap DVBv3 read SNR to DVBv5 CNR

Remove 'duplicate' DVBv3 read SNR implementation and return value,
calculated already by DVBv5 CNR, from the cache.

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