Remove Git Annex

Description

Git Annex is replaced by LFS and just a burden to support, let's remove it.

Proposal

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

Links / references

cc @dzaporozhets @marin @stanhu @DouweM @mydigitalself