[OVN] Remove session check in update_network_postcommit

Since [1], when a segment is deleted because the network is before,
the segment event handler method ``_handle_segment_change`` does not
call ``_notify_mechanism_driver_for_segment_change`` and thus the
check performed in ``OVNMechanismDriver.update_network_postcommit``
is not needed anymore.

[1]https://review.opendev.org/c/openstack/neutron/+/786373

Conflicts:
    neutron/plugins/ml2/drivers/ovn/mech_driver/mech_driver.py

Closes-Bug: #1739798
Change-Id: I4bb22a0a0a233609a4d23af55a050356049eb214
(cherry picked from commit 3202a5c)

Author: ralonsoh

neutron/plugins/ml2/drivers/ovn/mech_driver/mech_driver.py

@@ -47,7 +47,6 @@
 from neutron.common.ovn import exceptions as ovn_exceptions
 from neutron.common.ovn import extensions as ovn_extensions
 from neutron.common.ovn import utils as ovn_utils
-from neutron.common import utils
 from neutron.conf.plugins.ml2.drivers.ovn import ovn_conf
 from neutron.db import ovn_hash_ring_db
 from neutron.db import ovn_revision_numbers_db
@@ -620,10 +619,6 @@ def update_network_postcommit(self, context):
         network state.  It is up to the mechanism driver to ignore
         state or state changes that it does not know or care about.
         """
-        # FIXME(lucasagomes): We can delete this conditional after
-        # https://bugs.launchpad.net/neutron/+bug/1739798 is fixed.
-        if utils.is_session_active(context._plugin_context.session):
-            return
         self._ovn_client.update_network(
             context._plugin_context, context.current,
             original_network=context.original)