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/macros/macrosplugin.h

@@ -39,7 +39,6 @@ public:
     ~MacrosPlugin() final;
 
     bool initialize(const QStringList &arguments, QString *errorMessage) final;
-    void extensionsInitialized() final {}
 
 private:
     class MacrosPluginPrivate *d = nullptr;