Stop creating SSH keys for Geo secondaries

In %10.0 we stopped generating SSH keys for most GitLab EE installations. Only Geo secondaries generate these keys now.

Once https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3116 is merged, we can stop generating SSH keys for the secondaries as well. SSH repo sync will be deprecated and anyone wanting to continue using it will have to manage the SSH setup manually.