Remove another hand written API model.

There are still two more of these that don't show up in openapi.json
because they don't show up in HTTP routes (only in the websocket):

* GameUpdateEvents
* FrozenCombat

I'm not sure if there's a way to forcibly include those in the
openapi.json, if I should add a no-op API to force it to happen, or if I
should just ignore it. For now I'm going with option 3.

game/server/game/models.py

@@ -25,7 +25,7 @@ class GameJs(BaseModel):
     flights: list[FlightJs]
     threat_zones: ThreatZoneContainerJs
     navmeshes: NavMeshesJs
-    map_center: LeafletPoint
+    map_center: LeafletPoint | None
 
     class Config:
         title = "Game"