Remove a deprecated feature of GitLab

> We are also removing Wall from projects because we feel that the time needed for developing and keeping bug free is not justified if compared to the usefulness of the feature.
[GitLab Blog | GitLab 7.0 released with drag and drop images and performance improvements](https://about.gitlab.com/2014/06/22/gitlab-7-dot-0-released/)

closes https://github.com/progit/progit2-ja/issues/18