This mechanism driver was deprecated several cycles ago and we've not
used it in our testing for some time.
It's being removed from neutron[1] now so it's time to clean up
the unused logic.
[1] https://review.opendev.org/c/openstack/neutron/+/927216
Change-Id: I78bb7f9d49577b8556ea93067399f19b21a060c0