Remove Git Annex
Git Annex is replaced by LFS and just a burden to support, let's remove it.
We remove it with GitLab 9.0. Announce with 8.17.
deprecation warning in 8.17 blog post
~Platform removed from GitLab master
removed from Omnibus
~Documentation migration guide "From Annex to LFS" => https://gitlab.com/gitlab-org/gitlab-ee/issues/1693