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
diff --git a/doc/source/overview.rst b/doc/source/overview.rst
index 4384081..81e58a3 100644
--- a/doc/source/overview.rst
+++ b/doc/source/overview.rst
@@ -56,7 +56,7 @@
 -----------------
 
 -  Neutron: A basic configuration approximating the original FlatDHCP
-   mode using linuxbridge or OpenVSwitch.
+   mode using OpenVSwitch.
 
 Services
 --------