code: public function removeFlash($key)
      {
          $counters = $this->get($this->flashParam, []);
          $value = isset($_SESSION[$key], $counters[$key]) ? $_SESSION[$key] : null;
          unset($counters[$key], $_SESSION[$key]);
          $_SESSION[$this->flashParam] = $counters;
  
          return $value;
      }
docs: Removes a flash message.
  @param string $key the key identifying the flash message. 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.
  @return mixed the removed flash message. Null if the flash message does not exist.
  @see getFlash()
  @see setFlash()
  @see addFlash()
  @see removeAllFlashes()