Remove Deprecated NavigationExperimental

Summary:
Now that there are a number of good navigation solutions provided by the community, we are ready to remove NavigationExperimental from the RN core. The latest navigation doc explains the available options pretty well: http://facebook.github.io/react-native/docs/navigation.html . We should also add a mention to Airbnb's new native-navigation.

For anybody who continues to rely on it, it is recommended to migrate to React Navigation, which will be maintained over the long-term. For those who cannot migrate yet, it is possible to copy this code into your app.
Closes #13066

Differential Revision: D4757539

Pulled By: ericvicenti

fbshipit-source-id: 949d9b33f188584fb095155fa67d3ce24beba29f

Author: Eric Vicenti