Remove duplicated offline-plugin (hemanth#36)

The [offline-plugin](https://github.com/NekR/offline-plugin) was duplicated on the Tool section

README.md

@@ -150,4 +150,3 @@ _Source:_ [Google Developers - Progressive Web Apps](https://developers.google.c
 * [`fetch-manifest-json`](https://github.com/hemanth/fetch-manifest-json): Fetch the `mainfest.json` from an URL.
 * [`manifest-json`](https://www.npmjs.com/package/manifest-json): CLI tool for creating `mainfest.json`.
 * [`PWAify`](https://github.com/vladikoff/PWAify): CLI tool to convert your PWA into a cross-platform desktop app.
-* [`offline-plugin`](https://github.com/NekR/offline-plugin): Offline plugin (ServiceWorker, AppCache) for webpack.