Remove obsolete mailroom config update

Investigating the list of MRs for #545 (closed) , I noticed that https://dev.gitlab.org/cookbooks/chef-repo/merge_requests/2026 is closed. I also noticed that its changes seem to have been rolled into https://dev.gitlab.org/cookbooks/chef-repo/merge_requests/2218/diffs

This is fine, although it means we have to be sure that mailroom won't restart in gprd until we hit phase 7. It does mean that we don't need to list this MR any more, meaning only one MR is missing from the complete list