Can't remove current roles in ZMI

I got bitten by this as well. Changing the title of that button and adding a note in this form seems a good solution to me.

Done on Products.PluggableAuthService trunk in r120252.
Merged to the 1.6 branch in r120253.

For the record I will list some code i have now added to a client project to remove an old no longer needed role; may be handy for others:

    role = 'Moderator'

    # First the portal role manager.
    PAS = getToolByName(site, 'acl_users')
    rolemanager = PAS.portal_role_manager
    try:
        rolemanager.removeRole(role)
    except KeyError:
        pass

    # Then the roles in the root of the site.
    data = list(site.__ac_roles__)
    try:
        data.remove(role)
    except:
        logger.info('Old Moderator role was already removed.')
        return
    data.sort()
    site.__ac_roles__ = tuple(data)
    logger.info('Removed old Moderator role')