[environment] Remove environment variable whitelisting.
[environment] Remove environment variable whitelisting.

This causes more trouble than it's worth.

R=dnj@chromium.org, nodir@chromium.org, phajdan.jr@chromium.org, vadimsh@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2712543002

luci/recipes-py,luci/recipes-py