[Urgent] fix error when import config

1. What is the problem
The patch that refactors neutron ml2 config opts to be in
neutron/conf/plugins/ml2 is merged [1], making unit tests fail.

2. What is the solution for the problem
Delete the line which import neutron.plugins.ml2.config

3. What features need to be implemented to the Tricircle to
realize the solution
N/A

Change-Id: Ia4c8de9ac0dc66541c194588a8c51a489078d909

tricircle/network/central_trunk_plugin.py

@@ -18,7 +18,6 @@ import six
 
 from oslo_log import log
 
-from neutron.plugins.ml2 import config  # noqa
 from neutron.services.trunk import exceptions as trunk_exc
 from neutron.services.trunk import plugin
 from neutron_lib.plugins import directory