Remove broken data fixtures

The data fixtures in the class LoadForumData are currently broken. The loader is using repository methods which do not exist (board repository findOneCategoryByName($boardName) and category repository findOneCategoryByName($categoryName)).

When trying to load the fixtures for all bundles by calling "app/console doctrine:fixtures:load" it will fail as soon as the forum bundle is activated.

As far as I can tell fixtures are not really required for the forum so I suggest to just remove the broken fixtures loader.