package.json: Drop core-js

starter-kit does not use it directly, and none of our derived projects
have it.

src/index.js

@@ -19,8 +19,6 @@
 
 import "./lib/patternfly/patternfly-4-cockpit.scss";
 
-import "core-js/stable";
-
 import React from 'react';
 import ReactDOM from 'react-dom';
 import { Application } from './app.jsx';