Uncaught SecurityError in Google iframe when unmounting

When the component gets unmounted (e.g., when navigating to a different page using react-router), an exception occurs:

```
Uncaught SecurityError: Blocked a frame with origin "https://www.google.com" from
accessing a frame with origin "localhost". The frame requesting access has a protocol
of "https", the frame being accessed has a protocol of "http". Protocols must match.
```

Possible solution is to remove the iframe container inserted by the reCAPTCHA script in `componentWillUnmount`.