Improve DEAD mission planning.

We don't need to include a SEAD flight in missions against EWRs or SAMs
that no longer have a radar.

Also plan DEAD missions against air defenses that have no radars.
Previously we would never finish killing launcher only sites (which
cannot defend any more, but are cheaper to return to working order than
a fully destroyed site) nor would we plan DEAD against IR SAMs or AAA.

game/theater/theatergroundobject.py

@@ -137,7 +137,7 @@ class TheaterGroundObject(MissionTarget):
         return False
 
     @property
-    def has_radar(self) -> bool:
+    def has_alive_radar(self) -> bool:
         """Returns True if the ground object contains a unit with radar."""
         for group in self.groups:
             for unit in group.units: