Debugger: Do not send empty message on shutdown

This triggered a soft assert in DebuggerEngine::showMessage()
and cleaning up the status bar is not necessary at all.

Change-Id: I12d3d5cbc79f178af58ecb0a5c7a3130c880bdad
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/qml/qmlengine.cpp

@@ -570,7 +570,6 @@ void QmlEngine::shutdownEngine()
     stopApplicationLauncher();
 
     notifyEngineShutdownFinished();
-    showMessage(QString(), StatusBar);
 }
 
 void QmlEngine::setupEngine()