Issue #1988666 by Crell, kim.pepper: Cleanup Controllers and Enhancers.

core/core.services.yml

core/lib/Drupal/Core/Controller/AjaxController.php

core/lib/Drupal/Core/Controller/ControllerInterface.php

core/lib/Drupal/Core/Controller/ControllerResolver.php

core/lib/Drupal/Core/Controller/DialogController.php

core/lib/Drupal/Core/Controller/ExceptionController.php

core/lib/Drupal/Core/Controller/HtmlFormController.php

core/lib/Drupal/Core/Controller/HtmlPageController.php

core/lib/Drupal/Core/ControllerInterface.php

core/lib/Drupal/Core/Entity/HtmlEntityFormController.php

core/lib/Drupal/Core/Routing/Enhancer/AjaxEnhancer.php

...rupal/Core/Routing/Enhancer/ContentControllerEnhancer.php

core/lib/Drupal/Core/Routing/Enhancer/FormEnhancer.php

core/lib/Drupal/Core/Routing/Enhancer/ModalEnhancer.php

core/lib/Drupal/Core/Routing/Enhancer/PageEnhancer.php

...ib/Drupal/system/Tests/Routing/ControllerResolverTest.php

...ib/Drupal/system/Tests/System/ExceptionControllerTest.php