GitLab setup is quite stable these days thanks to the work of many
people and `coqbot`. We decided to keep CircleCI support for a while
as a safeguard in case something happened in the migration to GitLab,
but these days we are just wasting resources to them and to us. As I'm
afraid CircleCI won't scale for us, the time to remove it has arrived.

Still, CircleCI had some awesome functionality that GitLab's CI
doesn't offer yet, see the links at:

https://github.com/coq/coq/issues/6919#issuecomment-395885573

- https://gitlab.com/gitlab-org/gitlab-ce/issues/29347
- https://gitlab.com/gitlab-org/gitlab-ce/issues/35222
- https://gitlab.com/gitlab-org/gitlab-ce/issues/41947
- https://gitlab.com/gitlab-org/gitlab-ce/issues/47063