Remove dependency on Yeoman

Summary:
We stopped using Yeoman in #10786

I almost forgot to remove the now-unused dependency :)

**Test Plan**

- Published react-native to Sinopia
- Ran `react-native init MyApp`
- The app was generated correctly
- The app's node_modules folder doesn't contain Yeoman

Reviewed By: cpojer

Differential Revision: D4291619

fbshipit-source-id: 44c1ef8035fa2d8c40d4e8c505207245e1a95d3c