Remove unnecessary transform plugins for object spread to work

Created by: valscion

The babel-plugin-transform-object-rest-spread v6.19.0 update made the other two plugins unnecessary as it contains the fixes done in https://github.com/babel/babel/pull/4755

https://github.com/babel/babel/blob/v6.19.0/CHANGELOG.md

Ref https://github.com/facebookincubator/create-react-app/issues/904#issuecomment-261077538 and #927

Test plan

1. Use node v6.7.0

If you use nvm, this is how:

nvm install v6.7.0
nvm use v6.7.0

2. (Optional, I think) Use npm v2

npm install -g npm@2.x

3. Modify App.test.js

Modify packages/react-scripts/template/src/App.test.js to look like this:

import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';

it('renders without crashing', () => {
  const fn = ({ a, ...otherProps }) => otherProps;

  fn({ a: 1, b: 2 });

  console.log(fn({ a: 1, b: 2 }));

  const div = document.createElement('div');
  ReactDOM.render(<App />, div);
});

4. Install all the packages and run the test script

npm install
npm test