Drop log level to info for Qt console shutdown

At present, shutting down the Qt console displays this message in the terminal:

```
[IPythonQtConsoleApp] WARNING | shutdown: {'status': 'ok', 'restart': False}
```

I don't think this needs to be a warning - it's responding correctly to a user action. Info messages are not shown by default, so this change silences the message in the default case.