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/Traits/Buildings/Bridge.cs

@@ -325,8 +325,8 @@ namespace OpenRA.Mods.Common.Traits
 
 			// If this bridge repair operation connects two pathfinding domains,
 			// update the domain index.
-			var domainIndex = self.World.WorldActor.TraitOrDefault<DomainIndex>();
-			domainIndex?.UpdateCells(self.World, footprint.Keys);
+			var domainIndex = self.World.WorldActor.Trait<DomainIndex>();
+			domainIndex.UpdateCells(self.World, footprint.Keys);
 
 			if (LongBridgeSegmentIsDead() && !killedUnits)
 			{