Summary:
Remove the native iOS sticky headers implementation that has been replaced by the js Animated one. Also remove a line in JS that made sure we passed null to native so it did not use the native implementation.
**Test plan**
Made sure there were no more mentions of sticky / header in native ScrollView related code.
Tested that sticky headers still work :o
Closes https://github.com/facebook/react-native/pull/12696
Differential Revision: D4657391
Pulled By: ericvicenti
fbshipit-source-id: 16324a45ca4ce5cd143293c61394a0fa7ad0c4a1
5353d39172