Stop publishing jest/preprocessor.js to npm

Summary:
Changelog:
[Breaking][General] Remove jest/preprocessor from the react-native package

Using `react-native/jest/preprocessor` outside of the React Native repo has been deprecated since D34820086 (https://github.com/facebook/react-native/commit/c1e9aa9a272aed3cba60c4aeff783eeb8bffce68). Here, we make this file private and stop publishing it to npm.

The supported method of using Jest in React Native projects is using the [Jest preset](https://jestjs.io/docs/tutorial-react-native) shipped as part of the `react-native` package.

NOTE: We could probably make some of the other files under `jest/` private as well, but we should go through a separate deprecation cycle with those, in case they are also being used externally by some projects. I'm leaving that for future followup at the moment.

Reviewed By: GijsWeterings

Differential Revision: D36132939

fbshipit-source-id: 300ffa6cf16232573cd3a79a34c14daaafd7aa43