run-swarmed.py: Remove --env-prefix flags

They were causing all commands to fail with:
swarming: googleapi: Error 400: request.env_prefixes[u'PATH'] conflicts with pool's template

Was able to run base_unittests without them, so guessing they are no
longer needed.

Change-Id: If7a1992cb98ac96e056be0565b9d0430e498eb8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2111652Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#752048}

tools/run-swarmed.py

@@ -115,14 +115,6 @@ def _Spawn(args):
       vpython_native_pkg,
       '--cipd-package',
       vpython_pkg,
-      '--env-prefix',
-      'PATH=.swarming_module',
-      '--env-prefix',
-      'PATH=.swarming_module/bin',
-      '--env-prefix',
-      'VPYTHON_VIRTUALENV_ROOT=.swarming_module_cache/vpython',
-  ]
-  trigger_args += [
       '--',
       '--test-launcher-summary-output=${ISOLATED_OUTDIR}/output.json',
       '--system-log-file=${ISOLATED_OUTDIR}/system_log']