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/tasklist/tasklistplugin.h

@@ -43,7 +43,6 @@ public:
     ~TaskListPlugin() final;
 
     bool initialize(const QStringList &arguments, QString *errorMessage) override;
-    void extensionsInitialized() override {}
 
     static bool loadFile(QString *errorString, const Utils::FilePath &fileName);