AP_HAL_SITL: remove unimplemented methods

libraries/AP_HAL_SITL/SITL_State.h

@@ -175,9 +175,6 @@ private:
     void _fdm_input_local(void);
     void _output_to_flightgear(void);
     void _simulator_servos(struct sitl_input &input);
-    void _simulator_output(bool synthetic_clock_mode);
-    uint16_t _airspeed_sensor(float airspeed);
-    uint16_t _ground_sonar();
     void _fdm_input_step(void);
 
     void wait_clock(uint64_t wait_time_usec);