Make dir sessions in runtime not make sense

[RodrigoDornelles]

Because redis session inherits the default session class, it ends up creating the session folder, this could be removed.

I'm in doubt about the best way to remove this abstraction would be unnecessary, as it's all tied to file sessions.

https://github.com/yiisoft/yii2/blob/36ef75338536e855432ae7bd4a24d455fde63f90/framework/web/Session.php#L379

public function setSavePath($value)
  {
      $path = Yii::getAlias($value);
      if (is_dir($path)) {
          session_save_path($path);
      } else {
          throw new InvalidArgumentException("Session save path is not a valid directory: $value");
      }
  }

[samdark]

Yes, that could be considered a bug.