Revert "Pass ZUUL_* env vars through tox"
This reverts commit b70bbeb5c7.
This is no longer needed with the dependent commit merged.
Change-Id: I22fad4995cb379009a1fac9be25dd580e1aacee5
Depends-On: Iacba1611e5c50fbccd6ae700b3456028b142fc5c
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