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_floatingip.py b/neutron_tempest_plugin/scenario/test_floatingip.py
index 39aa09d..e276a02 100644
--- a/neutron_tempest_plugin/scenario/test_floatingip.py
+++ b/neutron_tempest_plugin/scenario/test_floatingip.py
@@ -17,6 +17,7 @@
 
 from neutron_lib import constants as lib_constants
 from neutron_lib.services.qos import constants as qos_consts
+from neutron_lib.utils import test
 from tempest.common import utils
 from tempest.common import waiters
 from tempest.lib.common.utils import data_utils
@@ -151,7 +152,7 @@
 
     same_network = True
 
-    @common_utils.unstable_test("bug 1717302")
+    @test.unstable_test("bug 1717302")
     @decorators.idempotent_id('05c4e3b3-7319-4052-90ad-e8916436c23b')
     def test_east_west(self):
         self._test_east_west()
@@ -169,7 +170,7 @@
 
     same_network = False
 
-    @common_utils.unstable_test("bug 1717302")
+    @test.unstable_test("bug 1717302")
     @decorators.idempotent_id('f18f0090-3289-4783-b956-a0f8ac511e8b')
     def test_east_west(self):
         self._test_east_west()
@@ -212,7 +213,7 @@
     def resource_setup(cls):
         super(FloatingIPPortDetailsTest, cls).resource_setup()
 
-    @common_utils.unstable_test("bug 1815585")
+    @test.unstable_test("bug 1815585")
     @decorators.idempotent_id('a663aeee-dd81-492b-a207-354fd6284dbe')
     def test_floatingip_port_details(self):
         """Tests the following: