Provide an empty default IPlugin::extensionsInitialized()

And remove all empy re-implementations.

Change-Id: I19f0b4e55c042c96693ecb89766f97f0a97b54ae
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/qmlpreview/qmlpreviewplugin.h

@@ -64,7 +64,6 @@ public:
     ~QmlPreviewPlugin() override;
 
     bool initialize(const QStringList &arguments, QString *errorString) override;
-    void extensionsInitialized() override;
     ShutdownFlag aboutToShutdown() override;
     QVector<QObject *> createTestObjects() const override;