swarming_lib: Remove SWARMING_TASK_ID from cmds.

If SWARMING_TASK_ID is present when running swarming proxy commands,
the proxy will try to make use of it. This fails for swarming builds,
since the swarming build instance and lab swarming proxy instance are
fully independent.

TEST=Unittests + cros tryjob --swarming -g XX samus-release-tryjob

Change-Id: I1cf18ef83b3b9e442ba7d5da63002a6d62fdf258
Reviewed-on: https://chromium-review.googlesource.com/961532
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
(cherry picked from commit 4cb4f50661ba05e369653216c4f82d13026937ac)
Reviewed-on: https://chromium-review.googlesource.com/1038483
4 files changed