Move ScrollResponder.Mixin methods into ScrollView and Remove ScrollResponder.js

Summary:
The purpose of this diff is to move all of the ScrollResponder methods into ScrollView to delete ScrollResponder.Mixin.

NOTE: ScrollResponder.Mixin uses a variable named "state" but it does not use React state correctly. Instead of calling `setState()`, state is set using `this.state.item = 123` ([example](https://www.internalfb.com/intern/diffusion/FBS/browsefile/master/xplat/js/react-native-github/Libraries/Components/ScrollResponder.js?lines=315)).  This means these are not actually React state - these are functionally just variables. In this stack, these "state" items from ScrollResponder are turned into regular internal variables.

Changelog:
[General][Removed] Moved ScrollResponder.Mixin methods into ScrollView to Remove ScrollResponder.js

Reviewed By: lunaleaps, nadiia

Differential Revision: D20715880

fbshipit-source-id: 99441434a6dc1c8ff3f435e7d6ec2840821e4e05

Author: kacieb