[rotorcraft] prevent motor arming in kill mode

Reported by Alonso Acuna:

>Hello. I have configured my multirotor with ```<define name="MODE_MANUAL" value="AP_MODE_KILL"/>```

>When this mode is set either by the RC or because the RC is off it is still possible to arm the motors with the button in the GCS. I think this makes no sense and is not safe. One could assume that when in MODE_KILL there would be no way for the motors to start.

This fix should prevent that...
Not sure if it would impact some other use case, but I can't think of one...