Move ChildActivity handling into base Activity class.

OpenRA.Game/Traits/ActivityUtils.cs

@@ -51,10 +51,10 @@ namespace OpenRA.Traits
 			return act;
 		}
 
-		public static Activity SequenceActivities(Actor self, params Activity[] acts)
+		public static Activity SequenceActivities(params Activity[] acts)
 		{
 			return acts.Reverse().Aggregate(
-				(next, a) => { a.Queue(self, next); return a; });
+				(next, a) => { a.Queue(next); return a; });
 		}
 	}
 }