remove redundant tolist method invocation

ToList method invocation is redundant.

src/Silverback.Core/Util/EnumerableSortExtensions.cs

@@ -17,8 +17,7 @@ public static IEnumerable<T> SortBySortIndex<T>(this IEnumerable<T> items)
 
             return sortables.Where(sorted => sorted.SortIndex <= 0).Cast<T>()
                 .Union(notSortables)
-                .Union(sortables.Where(b => b.SortIndex > 0).Cast<T>())
-                .ToList();
+                .Union(sortables.Where(b => b.SortIndex > 0).Cast<T>());
         }
     }
 }