team merge subscribes/assigns bugs to ~registry

The code to delete a team really uses the merge to make the unwanted team disappear. Merge reassigned roles and ownership to the remaining team. In the case of delete though, the desired behaviour should be like deactivation. The following is a list of subscriptions or roles that we expect to delete to set to None
    pillar.bug_supervisor = None
    pillar.bug_contact = None
    structure.driver = None
    bug.assignee = None
    del bug.subscription
    del structuralsubscription
    del branchsubscription