Summary:
We no longer need to manually build JS bundle, everything should be integrated into Xcode.
Closes https://github.com/facebook/react-native/pull/5521
Reviewed By: svcscm
Differential Revision: D2860511
Pulled By: martinbigio
fb-gh-sync-id: fdde6b39aabccd5a39c140d431acbbbe5af71ec4