Remove usage of unittest2

from comments when it was last touched it looks like workarounds for
unittest2 might be able to be dropped.

related: https://github.com/mtreinish/stestr/pull/265

simplify the workaround logic around unittest2 TestCase logic

Change-Id: Ibac9d0c2fa2f30605dd44ee58b84946464ea6449
6 files changed