MDL-75862 backup: omit sesskey for continue buttons using get method.

It's automatically added by the rendered single_button instance when
using post method, so no need to add it unconditionally.

backup/util/ui/renderer.php

@@ -553,7 +553,8 @@ class core_backup_renderer extends plugin_renderer_base {
     }
 
     /**
-     * Displays a continue button
+     * Displays a continue button, overriding core renderer method of the same in order
+     * to override submission method of the button form
      *
      * @param string|moodle_url $url
      * @param string $method
@@ -566,7 +567,6 @@ class core_backup_renderer extends plugin_renderer_base {
         if ($method != 'post') {
             $method = 'get';
         }
-        $url->param('sesskey', sesskey());
         $button = new single_button($url, get_string('continue'), $method, true);
         $button->class = 'continuebutton';
         return $this->render($button);