Remove unused babel-loader from babel-preset-react-app

[tlrobinson]

As far as I can tell there's no reason for the webpack babel-loader to be a dependency of babel-preset-react-app. If you happen to use babel-preset-react-app without Webpack you get a warning like

warning "babel-preset-react-app > [email protected]" has unmet peer dependency "webpack@>=2".

There was another PR #6124 but it wasn't merged for some reason.

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[ianschmitz]

Thanks!