Remove public column from namespaces and forms

It's not needed because all group are public by default

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/3549

https://dev.gitlab.org/gitlab/gitlabhq/issues/2565

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/47

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1751