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/clangformat/clangformatplugin.h

@@ -35,7 +35,6 @@ class ClangFormatPlugin : public ExtensionSystem::IPlugin
     Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QtCreatorPlugin" FILE "ClangFormat.json")
 
     bool initialize(const QStringList &arguments, QString *errorString) final;
-    void extensionsInitialized() final {}
 };
 
 } // namespace ClangTools