Move external url listing to provider for plugin use (#12279)

MediaBrowser.Controller/Entities/BaseItem.cs

@@ -2497,11 +2497,6 @@ namespace MediaBrowser.Controller.Entities
             return new[] { Id };
         }
 
-        public virtual List<ExternalUrl> GetRelatedUrls()
-        {
-            return new List<ExternalUrl>();
-        }
-
         public virtual double? GetRefreshProgress()
         {
             return null;