Remove timeutils.clear_time_override in neutron

The clear_time_override function in timeutils was written as a
helper function to mock utcnow for unittests before 'mock' was
generally used. Now that we have mock and fixture, we no longer
need to use it.

Change-Id: I7baa623fd71a7d03acc61dd1f18287bb654539cd
Partial-Bug: #1266962

neutron/tests/unit/services/metering/agents/test_metering_agent.py

@@ -130,7 +130,6 @@ class TestMeteringOperations(base.BaseTestCase):
 
         now = timeutils.utcnow()
         time_fixture = self.useFixture(utils_fixture.TimeFixture(now))
-        self.addCleanup(timeutils.clear_time_override)
 
         self.agent.routers_updated(None, ROUTERS)