Avoid redefining the lastWindowClosed() signal.

This prevents things like the following (from qmlviewer):
QMetaObject::indexOfSignal: signal lastWindowClosed() from QGuiApplication redefined in QApplication

Change-Id: I4b30235e379aedaa913ea30f05daac7079f285e9
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

src/widgets/kernel/qapplication.h

@@ -242,7 +242,6 @@ public:
 #endif
 
 Q_SIGNALS:
-    void lastWindowClosed();
     void focusChanged(QWidget *old, QWidget *now);
 #ifndef QT_NO_SESSIONMANAGER
     void commitDataRequest(QSessionManager &sessionManager);