Copter: add coax heli support

Motors should be attached to RCOutputs 1 and 2, Flaps should be connected
to RC outputs 3 and 4
Coax: remove unused servo3, servo4

Removed commented out code
Copter: moved coax servo set-up to AP_MotorCoax
Copter: removed unused servo3, servo4 from coax
Coax: servos to ch1, ch2, motors to ch3, ch4
Copter: coax allows ch5 and higher for do_set_servo cmd