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/qmlprojectmanager/qmlprojectplugin.h

@@ -41,7 +41,6 @@ public:
 
 private:
     bool initialize(const QStringList &arguments, QString *errorString) final;
-    void extensionsInitialized() final {}
 
     class QmlProjectPluginPrivate *d = nullptr;
 };