MDL-78425 admin: Point Clear theme caches to correct URL

* Plus remove unnecessary call to require_once() in theme/index.php

admin/classes/output/theme_selector.php

@@ -55,7 +55,7 @@ class theme_selector implements renderable, templatable {
         // Theme data used to populate cards and modal.
         $data->themes = $this->themedata;
         // Reset theme caches button.
-        $reseturl = new moodle_url('admin/themeselector.php', ['sesskey' => sesskey(), 'reset' => 1]);
+        $reseturl = new moodle_url('/admin/themeselector.php', ['sesskey' => sesskey(), 'reset' => 1]);
         $resetbutton = new \single_button($reseturl, get_string('themeresetcaches', 'admin'), 'post',
             \single_button::BUTTON_SECONDARY);
         $data->resetbutton = $resetbutton->export_for_template($output);

theme/index.php

@@ -22,7 +22,6 @@
  */
 
 require_once(__DIR__ . '/../config.php');
-require_once($CFG->libdir . '/adminlib.php');
 
 // Theme selector has been moved to admin/themeselector.php.
 redirect (new moodle_url('/admin/themeselector.php'));