QListWidget: remove unused member

Remove the 'dummy' member - it's not needed since ages.

Change-Id: I4869cf9153c892ea065340335ff7accd529a79c6
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/widgets/itemviews/qlistwidget.h

@@ -150,7 +150,6 @@ public:
 private:
     QListModel *listModel() const;
     int rtti;
-    QVector<void *> dummy;
     QListWidget *view;
     QListWidgetItemPrivate *d;
     Qt::ItemFlags itemFlags;