Fixed potential crash in BFAppLinkReturnToRefererController when it is initialized without navigation controller.

Looks like we are removing NSNotificationCenter observation, only in case when there was the controller was initialized with navigation controller - whilst this is not the only way you can initialize the `BFAppLinkReturnToRefererController`.

- Observe status bar and device orientation only when attached to the navigation controller
- Remove observe more efficiently (the `removeObserver:name:object:` is slower than just `removeObserver:`, as it performs unnecessary lookups)