Sub: Remove motors.set_air_density_ratio

Unused in AP_Motors6DOF

ArduSub/sensors.cpp

@@ -17,8 +17,6 @@ void Sub::read_barometer(void)
     baro_alt = barometer.get_altitude() * 100.0f;
     baro_climbrate = barometer.get_climb_rate() * 100.0f;
 
-    motors.set_air_density_ratio(barometer.get_air_density_ratio());
-
     sensor_health.depth = barometer.healthy(depth_sensor_idx);
 }