remove accidental commit of an if-statement

Author: corvinsz

src/MaterialDesignThemes.Wpf/TreeListView.cs

@@ -99,10 +99,7 @@ private void TreeListView_SelectionChanged(object sender, SelectionChangedEventA
         // Add newly selected
         foreach (var item in e.AddedItems)
         {
-            if (!SelectedItems.Contains(item))
-            {
-                SelectedItems.Add(item);
-            }
+            SelectedItems.Add(item);
         }
     }