Remove example extensions that depend on npm

Several of the examples here pull in dependencies for stuff like react, babel, webpack, eslint. They are useful to show how WebExtensions can integrate these technologies, but I think they are outside the core scope of this repo, which is to show how to use the WebExtension APIs.

They are still useful and if they were zero-cost, then I'd be happy to keep them, but they are a significantly greater maintenance burden than extensions that just use the WebExtension APIs. So I'd propose we just remove these examples.

This applies to:

• https://github.com/mdn/webextensions-examples/tree/master/eslint-example
• https://github.com/mdn/webextensions-examples/tree/master/mocha-client-tests
• https://github.com/mdn/webextensions-examples/tree/master/react-es6-popup
• https://github.com/mdn/webextensions-examples/tree/master/store-collected-images
• https://github.com/mdn/webextensions-examples/tree/master/webpack-modules

Start by reviewing the five named example directories: eslint-example, mocha-client-tests, react-es6-popup, store-collected-images, and webpack-modules. Confirm their npm-dependent contents and remove the examples so the repository retains only examples focused on the WebExtension APIs.