Revert "Have some activities count as idle activities"

This reverts commit ae111248f3.

OpenRA.Game/Activities/Activity.cs

@@ -152,7 +152,6 @@ namespace OpenRA.Activities
 			set { NextActivity = value; }
 		}
 
-		public bool IsIdle { get; protected set; }
 		public bool IsInterruptible { get; protected set; }
 		public bool IsCanceled { get { return State == ActivityState.Canceled; } }