Remove part related to Assetic which is no longer used

Since #387 Assetic is no longer used to manage assets

Author: apetitpa

CONTRIBUTING.md

@@ -5,17 +5,3 @@ The Symfony Demo application is an open source project. Contributions made by
 the community are welcome. Send us your ideas, code reviews, pull requests and
 feature requests to help us improve this project. All contributions must follow
 the [usual Symfony contribution requirements](http://symfony.com/doc/current/contributing/index.html).
-
-Web Assets Management
----------------------
-
-This project manages its web assets in a special way to allow them to work
-without configuring any option, installing any tool or executing any command.
-If your contribution changes CSS styles or JavaScript code in any way, make
-sure to regenerate the `web/css/app.css` and `web/js/app.js` files. To do so,
-uncomment the Assetic blocks in the `app/Resources/views/base.html.twig` and
-execute the following command:
-
-```bash
-$ php bin/console assetic:dump --no-debug
-```