Handle latest testtools tearDown checks

Latest testtools checks that you only call tearDown once (it also checks
that setUp is only called once). Zuul's test framework was calling
tearDown from a cleanup which resulted in tearDown being called twice.
Once by the test framework and once by zuul's testclasses cleanups.

Just remove the explicit tearDown call in the cleanups and let the
larger python tooling call tearDown for us.

Change-Id: Id8eaa9da5153fdfe9b9f852d2c57838736cb7040