Chore: Remove unused babel-polyfill

[tonyjin]

With Node v8+ we no longer need babel-polyfill for Webpack.

[boxcla]

Verified that @tonyjin has signed the CLA. Thanks for the pull request!

[priyajeet]

Probably add these in package.json

"devEngines": {
    "node": ">=8.x",
    "npm": ">=3.x",
    "yarn": ">=1.0.0"
  },

[tonyjin]

@priyajeet you don't strictly need yarn right? I'll add node & npm. Where's the documentation on this?Couldn't find anything except for github PRs with a quick search of "npm devEngines" or "package.json devEngines"

[tonyjin]

Decided to remove devEngines for now since it doesn't seem to be an official option in npm or yarn, just engines. devEngines seems to be a React-only feature and they have a custom post-install hook that checks this: https://github.com/facebook/fbjs/blob/master/packages/fbjs-scripts/node/check-dev-engines.js#L29-L61