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" => #1693 (closed)