React Warning when remove() OB in componentWillUnmount

Is this a bug?

```
  // get a warning about forceUpdate of an unmounted component
  componentWillUnmount() {
    ob.remove();
  }
```

```
  // no warning
  componentWillUnmount() {
    ob.atomically().remove().commit({notify: false});
  }
```

we could fix this by checking c.isMounted() in the render function but is this a case where the correct thing to do is the second approach?
