Comment 11 for bug 1844168

Reviewed: https://review.opendev.org/703067
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=197c57cf23728614cf37c16ed3f6080e19fdacb3
Submitter: Zuul
Branch: stable/stein

commit 197c57cf23728614cf37c16ed3f6080e19fdacb3
Author: LIU Yulong <email address hidden>
Date: Tue Sep 17 00:58:08 2019 +0800

    Stop verifying unique external_net_id

    This cherry-pick is modified to omit the RPC modification of the
    original patch. Instead it only drops the code that would generate an
    exception if there are two different external networks. Tests are
    amended accordingly.

    Original patch description:

    Remove get_external_network_id for router

    L3 agent supports multiple external networks from a long
    time ago, so remove this RPC call since it is not used.
    According to codesearch of [1] and [2], we just remove
    neutron built-in L3 agent RPC. For neutron server side
    or RPC callback classes, the function is still remained.

    [1] http://codesearch.openstack.org/?q=get_external_network_id
    [2] http://codesearch.openstack.org/?q=L3RpcCallback

    Conflicts:
            neutron/agent/l3/agent.py

    Change-Id: I764423e175d6e82729a647e415a9f267f495916f
    Closes-Bug: #1844168
    (cherry picked from commit f51e5ce9241c9bbd0e2ed786f41dc11ed0146b88)