Remove NavigatorIOS

Summary:
Legacy navigator impl. There are other alternatives that should be used instead.

Part of the slimmening effort as described here: react-native-community/discussions-and-proposals#6

Reviewed By: TheSavior

Differential Revision: D9677824

fbshipit-source-id: 24ae500751d2a8c398f246d36604a58f0b3c113b

Author: fkgozali