Improve prioritization of garrison targeting.

Garrison groups should be preferred with the following priority:

1. Groups blocking base capture
2. Groups at bases connected to an active front line
3. Rear guard units

Previously they were being prioritized based on the distance to the
closest friendy control point, which is similar to this but an
aggressively placed carrier could throw it off.

game/commander/objectivefinder.py

@@ -65,14 +65,6 @@ class ObjectiveFinder:
 
                 yield ground_object
 
-    def threatening_vehicle_groups(self) -> Iterator[VehicleGroupGroundObject]:
-        """Iterates over enemy vehicle groups near friendly control points.
-
-        Groups are sorted by their closest proximity to any friendly control
-        point (airfield or fleet).
-        """
-        return self._targets_by_range(self.enemy_vehicle_groups())
-
     def enemy_ships(self) -> Iterator[NavalGroundObject]:
         for cp in self.enemy_control_points():
             for ground_object in cp.ground_objects: