backbone js book was duplicated in js and backbonejs section so I removed it from js section (#3460)

free-programming-books.md

@@ -1518,7 +1518,6 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Book of Modern Frontend Tooling](https://github.com/tooling/book-of-modern-frontend-tooling)
 * [Crockford's JavaScript](http://www.crockford.com/javascript/) - Douglas Crockford
 * [Dev Docs](http://devdocs.io)
-* [Developing Backbone.js Applications](http://addyosmani.github.io/backbone-fundamentals/) - Addy Osmani
 * [Directory of free Javascript ebooks](http://jsbooks.revolunet.com)
 * [Eloquent JavaScript 2nd edition](http://eloquentjavascript.net) - Marijn Haverbeke
 * [Exploring ES6](http://exploringjs.com/es6/) - Dr. Axel Rauschmayer