Removes use of timeutils.set_time_override

The set_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: I9a0727edab12ccd5f1e4eb4f5f62d588f5a0faee
Partial-Bug: #1266962