[TASK] Remove legacy code

Classes/Controller/ConfigurationController.php

Classes/Controller/LayoutController.php

Classes/Controller/MailController.php

Classes/Controller/NewsletterController.php

Classes/Controller/QueueController.php

Classes/Service/BackendService.php

Classes/UserFunc/ExtensionConfiguration.php

Classes/Utility/ExtensionUtility.php

Classes/XClass/Form/FormEditorController.php

Resources/Private/Layouts/Default.html

Resources/Private/Layouts/Newsletter.html

Resources/Private/Partials/Module/DocHeader.html