Improve Aircraft firstTick code

- Remove ReserveSpawnBuilding:
Only used in one place, and removing it
avoids a double GetActorBelow() look-up.

- Remove FallsToEarth check form Aircraft.firstTick:
Aircraft triggers UnReserve() on actor disposal,
so this work-around should no longer be necessary.

OpenRA.Mods.Common/Traits/Air/Aircraft.cs

@@ -321,16 +321,12 @@ namespace OpenRA.Mods.Common.Traits
 			{
 				firstTick = false;
 
-				// TODO: Aircraft husks don't properly unreserve.
-				if (self.Info.HasTraitInfo<FallsToEarthInfo>())
-					return;
-
-				ReserveSpawnBuilding();
-
 				var host = GetActorBelow();
 				if (host == null)
 					return;
 
+				MakeReservation(host);
+
 				if (Info.TakeOffOnCreation)
 					self.QueueActivity(new TakeOff(self));
 			}
@@ -478,16 +474,6 @@ namespace OpenRA.Mods.Common.Traits
 				.FirstOrDefault(a => a.Info.HasTraitInfo<ReservableInfo>());
 		}
 
-		protected void ReserveSpawnBuilding()
-		{
-			// HACK: Not spawning in the air, so try to associate with our spawner.
-			var spawner = GetActorBelow();
-			if (spawner == null)
-				return;
-
-			MakeReservation(spawner);
-		}
-
 		public void MakeReservation(Actor target)
 		{
 			UnReserve();