The openstack-security mailing list is officially closing, and wants
future attempts at posting to end up on openstack-discuss instead:
http://lists.openstack.org/pipermail/openstack-security/2021-June/006077.html
This was also the only remaining user of the notify-impact Gerrit
hook, so we can stop installing/running it.
Change-Id: Id60b781beb072366673b32326e32fd79637c1219