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/lib/neutron b/lib/neutron
index bcef8a5..69ff212 100644
--- a/lib/neutron
+++ b/lib/neutron
@@ -241,8 +241,7 @@
 # If using VLANs for tenant networks, or if using flat or VLAN
 # provider networks, set in ``localrc`` to the name of the physical
 # network, and also configure ``OVS_PHYSICAL_BRIDGE`` for the
-# openvswitch agent or ``LB_PHYSICAL_INTERFACE`` for the linuxbridge
-# agent, as described below.
+# openvswitch agent, as described below.
 #
 # Example: ``PHYSICAL_NETWORK=default``
 PHYSICAL_NETWORK=${PHYSICAL_NETWORK:-public}
@@ -257,18 +256,6 @@
 # Example: ``OVS_PHYSICAL_BRIDGE=br-eth1``
 OVS_PHYSICAL_BRIDGE=${OVS_PHYSICAL_BRIDGE:-br-ex}
 
-# With the linuxbridge agent, if using VLANs for tenant networks,
-# or if using flat or VLAN provider networks, set in ``localrc`` to
-# the name of the network interface to use for the physical
-# network.
-#
-# Example: ``LB_PHYSICAL_INTERFACE=eth1``
-if [[ $Q_AGENT == "linuxbridge" && -z ${LB_PHYSICAL_INTERFACE} ]]; then
-    default_route_dev=$( (ip route; ip -6 route) | grep ^default | head -n 1 | awk '{print $5}')
-    die_if_not_set $LINENO default_route_dev "Failure retrieving default route device"
-    LB_PHYSICAL_INTERFACE=$default_route_dev
-fi
-
 # With the openvswitch plugin, set to True in ``localrc`` to enable
 # provider GRE tunnels when ``ENABLE_TENANT_TUNNELS`` is False.
 #
@@ -889,10 +876,6 @@
         neutron_ovs_base_cleanup
     fi
 
-    if [[ $Q_AGENT == "linuxbridge" ]]; then
-        neutron_lb_cleanup
-    fi
-
     # delete all namespaces created by neutron
     for ns in $(sudo ip netns list | grep -o -E '(qdhcp|qrouter|fip|snat)-[0-9a-f-]*'); do
         sudo ip netns delete ${ns}