Clean up usage of DomainIndex

- When a path search is being performed the path search will not attempt route to inaccessible cells, so domain index checks to avoid inaccessible cells in the search predicate are redundant and can be removed.
- DomainIndex is a required world trait, so we don't need to use TraitOrDefault and therefore can avoid dealing with the null case.

OpenRA.Mods.Common/Activities/Move/MoveAdjacentTo.cs

@@ -22,7 +22,6 @@ namespace OpenRA.Mods.Common.Activities
 	public class MoveAdjacentTo : Activity
 	{
 		protected readonly Mobile Mobile;
-		readonly DomainIndex domainIndex;
 		readonly Color? targetLineColor;
 
 		protected Target Target => useLastVisibleTarget ? lastVisibleTarget : target;
@@ -37,7 +36,6 @@ namespace OpenRA.Mods.Common.Activities
 			this.target = target;
 			this.targetLineColor = targetLineColor;
 			Mobile = self.Trait<Mobile>();
-			domainIndex = self.World.WorldActor.Trait<DomainIndex>();
 			ChildHasPriority = false;
 
 			// The target may become hidden between the initial order request and the first tick (e.g. if queued)
@@ -123,7 +121,7 @@ namespace OpenRA.Mods.Common.Activities
 				searchCells.Clear();
 				searchCellsTick = self.World.WorldTick;
 				foreach (var cell in CandidateMovementCells(self))
-					if (domainIndex.IsPassable(loc, cell, Mobile.Locomotor) && Mobile.CanEnterCell(cell))
+					if (Mobile.CanEnterCell(cell))
 						searchCells.Add(cell);
 			}