Remove "unstable_test" decorator local implementation

Since [1] this decorator is in neutron-lib and used in this
repository. This local implementation can be removed.

[1] https://review.opendev.org/#/c/560102/

Trivial-Fix

Change-Id: I3c5f135fafcc380918905d8af164b71504441dc0
diff --git a/neutron_tempest_plugin/scenario/test_multicast.py b/neutron_tempest_plugin/scenario/test_multicast.py
index d511b3b..a39a0c3 100644
--- a/neutron_tempest_plugin/scenario/test_multicast.py
+++ b/neutron_tempest_plugin/scenario/test_multicast.py
@@ -15,6 +15,7 @@
 
 import netaddr
 from neutron_lib import constants
+from neutron_lib.utils import test
 from oslo_log import log
 from tempest.lib.common.utils import data_utils
 from tempest.lib import decorators
@@ -229,7 +230,7 @@
         server['ssh_client'].execute_script(
             'echo "%s" > ~/multicast_traffic_receiver.py' % check_script)
 
-    @utils.unstable_test("bug 1850288")
+    @test.unstable_test("bug 1850288")
     @decorators.idempotent_id('113486fc-24c9-4be4-8361-03b1c9892867')
     def test_multicast_between_vms_on_same_network(self):
         """Test multicast messaging between two servers on the same network