[rilmodem] Disable cbs support

We don't know for sure if registering to listen CBS messages
causes unnecessary power consumption with our modem but since we are
not really supporting it it's better to disable the feature for now.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
[rilmodem] Do not handle unexisting response

Running disable-modem test case seems to cause assert triggering
in gril in developer mode. Since method seems to survive just fine
from handling of zero length command queue, removing the assert
helps testing in developer mode.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>