Replace our local types by flow-typed (#20320)

Summary:
These comments were confusing since we don't use flow-typed.

cc bvaughn.

I have replaced our local types by flow-typed, removed metro custom stub and deleted `flow-github` directory.

As result, createReactClass, PropTypes and Metro types won't be distributed to RN users. If you need it, you should relay on flow-typed.
Pull Request resolved: https://github.com/facebook/react-native/pull/20320

Reviewed By: TheSavior

Differential Revision: D13517157

Pulled By: rickhanlonii

fbshipit-source-id: e52c0b6114114a706f6152b2031c600e3dece0d2

package.json

@@ -104,7 +104,6 @@
     "android",
     "cli.js",
     "flow",
-    "flow-github",
     "init.sh",
     "scripts/ios-configure-glog.sh",
     "scripts/ios-install-third-party.sh",