Removed unnecessary lock from a aggregate test

Lock is unnecessary here, as the test is not adding
a host to an aggregate.Previoulsy this lock was
added to ensure the failure will not occur where
a host is added to an availability zone.
Reference:-
Iaf8a903d92c88c7c3092eaa00a9f7a96e6348ddc

Change-Id: I9d6d610302406d5aa7bee9135c27dbc83896e005
diff --git a/tempest/api/compute/admin/test_aggregates.py b/tempest/api/compute/admin/test_aggregates.py
index 362cf2d..98d2550 100644
--- a/tempest/api/compute/admin/test_aggregates.py
+++ b/tempest/api/compute/admin/test_aggregates.py
@@ -108,7 +108,6 @@
     @attr(type='gate')
     def test_aggregate_create_update_with_az(self):
         # Update an aggregate and ensure properties are updated correctly
-        self.useFixture(fixtures.LockFixture('availability_zone'))
         aggregate_name = data_utils.rand_name(self.aggregate_name_prefix)
         az_name = data_utils.rand_name(self.az_name_prefix)
         resp, aggregate = self.client.create_aggregate(aggregate_name, az_name)