MDL-82055 output: replace usage of named templatable interface.

It's not available during, and breaks, site installation. The output
class doesn't necessarily need it in 78db6bbce8, so switch to simple
templatable interface instead.

lib/outputcomponents.php

@@ -26,8 +26,7 @@
  * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 
- use core\output\local\action_menu\subpanel;
- use core\output\named_templatable;
+use core\output\local\action_menu\subpanel;
 
 defined('MOODLE_INTERNAL') || die();
 
@@ -4110,7 +4109,7 @@ class tabobject implements renderable, templatable {
  * @copyright 2015 Adrian Greeve <[email protected]>
  * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
-class context_header implements renderable, named_templatable {
+class context_header implements renderable, templatable {
 
     /**
      * @var string $heading Main heading.
@@ -4232,16 +4231,6 @@ class context_header implements renderable, named_templatable {
             'additionalbuttons' => $additionalbuttons ?? [],
         ];
     }
-
-    /**
-     * Get the template name.
-     *
-     * @param renderer_base $renderer Renderer.
-     * @return string
-     */
-    public function get_template_name(renderer_base $renderer): string {
-        return 'core/contextheader';
-    }
 }
 
 /**