code: public function removeAllFlashes()
      {
          $counters = $this->get($this->flashParam, []);
          foreach (array_keys($counters) as $key) {
              unset($_SESSION[$key]);
          }
          unset($_SESSION[$this->flashParam]);
      }
docs: Removes all flash messages.
  Note that flash messages and normal session variables share the same name space.
  If you have a normal session variable using the same name, it will be removed
  by this method.
  @see getFlash()
  @see setFlash()
  @see addFlash()
  @see removeFlash()