AP_Compass: remove HAL_COMPASS_DEFAULT define

this doesn't actually allow you to specify a default, despite the name
and a couple of hwdefs trying to use it that way.

This would have stopped being useful when we did the
compass-prority-table stuff - or before.

libraries/AP_Compass/AP_Compass.cpp

@@ -1353,11 +1353,6 @@ void Compass::_detect_backends(void)
     default:
         break;
     }
-
-#elif HAL_COMPASS_DEFAULT == HAL_COMPASS_NONE
-    // no compass, or only external probe
-#else
-#error Unrecognised HAL_COMPASS_TYPE setting
 #endif