AP_GPS: removed shadowed class variable _nav_setting

this caused a uBlox to incorrectly configure as "portable" instead of
airborne if the detection took more than 2 seconds. That led to
terrible performance.

libraries/AP_GPS/AP_GPS_Auto.h

@@ -45,7 +45,5 @@ private:
 
     static const prog_char          _mtk_set_binary[];
     static const prog_char          _sirf_set_binary[];
-
-    enum GPS_Engine_Setting         _nav_setting;
 };
 #endif