Retire deprecations for constants, exceptions, attributes

Subprojects have had enough time to switch to neutron-lib for
the deprecated items from these modules.

Change-Id: Ib2f4acf7e0153667389c57a52f2db33a23758b24
diff --git a/neutron/tests/tempest/api/test_dhcp_ipv6.py b/neutron/tests/tempest/api/test_dhcp_ipv6.py
index e136efa..bf45825 100644
--- a/neutron/tests/tempest/api/test_dhcp_ipv6.py
+++ b/neutron/tests/tempest/api/test_dhcp_ipv6.py
@@ -14,10 +14,10 @@
 #    under the License.
 
 import netaddr
+from neutron_lib import constants
 from tempest.lib import exceptions as lib_exc
 from tempest import test
 
-from neutron.common import constants
 from neutron.tests.tempest.api import base
 from neutron.tests.tempest import config