Remove anything that has accumulated in site-packages before running AU test.
Remove anything that has accumulated in site-packages before running AU test.

BUG=chromium:382624
TEST=Unittests/Tryjob/Ran full_release_test without dir locally

Change-Id: I6ac6de5921ffbd24dd4376e65e257d5a26140766
Reviewed-on: https://chromium-review.googlesource.com/207813
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Commit-Queue: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Tested-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite