Remove attached() definition from header.

The implementation of attached() method was removed in commit 'Fix SHM
drawing logic' (19260d98

) but the
definition in the header file was not.

Change-Id: Ib850bc7101a661882078be95011d75660f621622
Reviewed-by: Giulio Camuffo <giulio.camuffo@kdab.com>

src/client/qwaylandwindow_p.h

@@ -120,7 +120,6 @@ public:
     using QtWayland::wl_surface::attach;
     void attach(QWaylandBuffer *buffer, int x, int y);
     void attachOffset(QWaylandBuffer *buffer);
-    QWaylandBuffer *attached() const;
     QPoint attachOffset() const;
 
     using QtWayland::wl_surface::damage;