fix: Changes Map.Sector.Items to link list. (#1547)

### Summary

Eliminates `IPooledEnumerable<T>` and `eable.Free()` from `Map` for items. This drastically simplifies code that iterates in range, for example:

```cs
foreach (var item in m.GetItemsInRange(5))
{
}
```
The code above no longer requires an eable and calling `Free()`.

Projects/UOContent/Commands/SignParser.cs

@@ -91,9 +91,7 @@ namespace Server.Commands
 
         public static void Add_Static(int itemID, Point3D location, Map map, string name)
         {
-            var eable = map.GetItemsInRange(location, 0);
-
-            foreach (var item in eable)
+            foreach (var item in map.GetItemsInRange(location, 0))
             {
                 if (item is Sign && item.Z == location.Z && item.ItemID == itemID)
                 {