Don't reserve airfields/helipads for husks that just fall down

fixes #3376

OpenRA.Mods.RA/Air/Aircraft.cs

@@ -86,10 +86,8 @@ namespace OpenRA.Mods.RA.Air
 
 		protected void ReserveSpawnBuilding()
 		{
-			/* not spawning in the air, so try to assoc. with our afld. this is a hack. */
-
+			/* HACK: not spawning in the air, so try to assoc. with our afld. */
 			var afld = GetActorBelow();
-
 			if (afld == null)
 				return;
 

OpenRA.Mods.RA/Air/Helicopter.cs

@@ -106,6 +106,7 @@ namespace OpenRA.Mods.RA.Air
 			if (firstTick)
 			{
 				firstTick = false;
+				if (!self.HasTrait<FallsToEarth>()) // TODO: Aircraft husks don't properly unreserve.
 					ReserveSpawnBuilding();
 			}
 

OpenRA.Mods.RA/Air/Plane.cs

@@ -32,6 +32,7 @@ namespace OpenRA.Mods.RA.Air
 			if (firstTick)
 			{
 				firstTick = false;
+				if (!self.HasTrait<FallsToEarth>()) // TODO: Aircraft husks don't properly unreserve.
 					ReserveSpawnBuilding();
 			}
 		}