Remove dependency on Yeoman

Summary:
We stopped using Yeoman in https://github.com/facebook/react-native/pull/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

package.json

@@ -191,9 +191,7 @@
     "ws": "^1.1.0",
     "xcode": "^0.8.9",
     "xmldoc": "^0.4.0",
-    "yargs": "^6.4.0",
+    "yargs": "^6.4.0"
-    "yeoman-environment": "1.5.3",
-    "yeoman-generator": "0.21.2"
   },
   "devDependencies": {
     "babel-eslint": "^7.0.0",