The fact that we aren't dealing with the Django source tree anymore
allows us to drop several tearDown()/addCleanup() calls that were
concerned with removing apiece files/dirs/symlinks created by test
cases, as we are covered by the removal of the parent temporary tree
anyways.
Thanks Tim Graham for advice and review.
Backport of bb7bb379e8cd91a91336946829519d64e919a1d2 from master