MDL-78179 badges: Clean SESSION when user disconnect backpack

The token information was not being properly cleared from the $SESSION.
As a result, certain errors were occurring when users attempted to
reconnect within the same session.

badges/classes/backpack_api.php

@@ -640,6 +640,8 @@ class backpack_api {
         $DB->delete_records('badge_external', array('backpackid' => $backpackid));
         $DB->delete_records('badge_backpack', array('userid' => $userid));
         $badgescache->delete($userid);
+        $this->clear_system_user_session();
+
         return true;
     }