AP_Airspeed: fix compiler warnings

In file included from
../../libraries/AP_Airspeed/AP_Airspeed_SDP3X.cpp:21:
../../libraries/AP_Airspeed/AP_Airspeed_SDP3X.h:57:11: warning: private
field '_temperature' is not used [-Wunused-private-field]
    float _temperature;
          ^
../../libraries/AP_Airspeed/AP_Airspeed_SDP3X.h:58:11: warning: private
field '_pressure' is not used [-Wunused-private-field]
    float _pressure;

libraries/AP_Airspeed/AP_Airspeed_SDP3X.h

@@ -54,8 +54,6 @@ private:
 
     float _temp;
     float _press;
-    float _temperature;
-    float _pressure;
     uint16_t _temp_count;
     uint16_t _press_count;
     float _temp_sum;