Remove support for Behat Sf Extension

The problem here is that the symfony extension for behat is not
maintained and not compatible with new versions of Symfony. It has been
officialized here:
Behat/Symfony2Extension#152 (comment)

The new correct way to use behat with Symfony is to use the package
symfony extension from the FriendsOfBehat organization.
- https://github.com/FriendsOfBehat/SymfonyExtension
- https://github.com/symfony/recipes-contrib/tree/master/friends-of-behat/symfony-extension/2.0

Author: Nek-