Remove Neutron Linux Bridge agent code

Linux Bridge agent support was removed in the Neutron
master branch in [0], let's remove any code here now
as well since it will just fail.

[0] https://review.opendev.org/c/openstack/neutron/+/927216

Change-Id: Idffa6579000322acfeb860189fb83a317d56bb4e
7 files changed