Don't uninstall flow tables in setup_dvr_flows_on_integ_br

This is a revise for I7b24a159962af7b58c096a1b2766e2169e9f8aed

Br-int's flow tables are already uninstalled in setup_integration_br.
And setup_integration_br will install some default flows. If we still
uninstall flow tables of br-int in setup_dvr_flows_on_integ_br, these
default flow tables will be missing.

Closes-Bug: #1775146
Change-Id: I71c1f9034dfc913b9e9ae17cc8f6bd084c9ee580
(cherry picked from commit 760870b6c2)

neutron/plugins/ml2/drivers/openvswitch/agent/ovs_dvr_neutron_agent.py

@@ -16,7 +16,6 @@
 import sys
 
 from neutron_lib import constants as n_const
-from oslo_config import cfg
 from oslo_log import log as logging
 import oslo_messaging
 from oslo_utils import excutils
@@ -29,9 +28,6 @@ from neutron.plugins.ml2.drivers.openvswitch.agent.common import constants
 
 LOG = logging.getLogger(__name__)
 
-cfg.CONF.import_group('AGENT', 'neutron.plugins.ml2.drivers.openvswitch.'
-                      'agent.common.config')
-
 
 # A class to represent a DVR-hosted subnet including vif_ports resident on
 # that subnet
@@ -142,7 +138,6 @@ class OVSDVRNeutronAgent(object):
         self.dvr_mac_address = None
         if self.enable_distributed_routing:
             self.get_dvr_mac_address()
-        self.conf = cfg.CONF
 
     def setup_dvr_flows(self):
         self.setup_dvr_flows_on_integ_br()
@@ -207,10 +202,6 @@ class OVSDVRNeutronAgent(object):
 
         LOG.info(_LI("L2 Agent operating in DVR Mode with MAC %s"),
                  self.dvr_mac_address)
-        # Remove existing flows in integration bridge
-        if self.conf.AGENT.drop_flows_on_start:
-            self.int_br.uninstall_flows()
-
         # Add a canary flow to int_br to track OVS restarts
         self.int_br.setup_canary_table()