QToolBarExtension: remove unused member 'orientation'

The orientation is implicitly stored by which icon is used.

Found by Clang:
  qtoolbarextension_p.h:57:21: error: private field 'orientation' is not used [-Werror,-Wunused-private-field]

Change-Id: I82f8b8009b48d41fd2beb95d6107e505f9d4e835
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/widgets/widgets/qtoolbarextension_p.h

@@ -54,7 +54,6 @@ QT_BEGIN_NAMESPACE
 class Q_AUTOTEST_EXPORT QToolBarExtension : public QToolButton
 {
     Q_OBJECT
-    Qt::Orientation orientation;
 
 public:
     explicit QToolBarExtension(QWidget *parent);