AP_AHRS: fix use of undefined macro

../../libraries/AP_AHRS/examples/AHRS_Test/AHRS_Test.cpp:63:5: warning: "WITH_GPS" is not defined [-Wundef]
 #if WITH_GPS
     ^

g_gps was not even declared so remove it.

libraries/AP_AHRS/examples/AHRS_Test/AHRS_Test.cpp

@@ -60,9 +60,6 @@ void loop(void)
         heading = compass.calculate_heading(ahrs.get_rotation_body_to_ned());
         // read compass at 10Hz
         last_compass = now;
-#if WITH_GPS
-        g_gps->update();
-#endif
     }
 
     ahrs.update();