Remove use_mox directive

As of horizon 14.0.0.0b2 or later the use_mox directive is unecessary so we can remove it.

Change-Id: I5866a1b8618c6cd6389fce70ed8d519cd77d1075

monitoring/test/helpers.py

@@ -43,7 +43,6 @@ class MonitoringTestsMixin(object):
 @unittest.skipIf(os.environ.get('SKIP_UNITTESTS', False),
                  "The SKIP_UNITTESTS env variable is set.")
 class TestCase(MonitoringTestsMixin, helpers.TestCase):
-    use_mox = False
     pass