Revisit the decision to allow webpack analyzer tools

Created by: gaearon

I haven't been following https://github.com/facebook/create-react-app/pull/3945 closely and it just got merged. But I'm not so sure this is a great idea (see https://github.com/facebook/create-react-app/issues/1858#issuecomment-358497143 for why).

I'll want to look at this again closer to shipping 2.x and decide whether to ship or remove this.