Fix duplicate teardown to allow tox upgrade

When trying to use tox 1.7.2 we run into a second
tearDown call being made in the test as opposed to
just using Super's version of it.

This removes that call and seems to get everything
working under tox 1.7.2

Change-Id: I0a2674b7c1ba9589cab5e3b76777fce23b6601ad
Partial-Bug: 1348818