Revert of It's time to bring subprocess42 to depot_tools. (patchset #1 id:1 of...

Revert of It's time to bring subprocess42 to depot_tools. (patchset #1 id:1 of https://codereview.chromium.org/2291223003/ )

Reason for revert:
Speculative revert because it might be breaking presubmit in depot_tools.

Original issue's description:
> It's time to bring subprocess42 to depot_tools.
>
> The immediate need is to properly kill git fetch and its sub-spawned
> process from gclient.
>
> R=maruel@chromium.org, hinoka@chromium.org
> BUG=635641
>
> Committed: https://chromium.googlesource.com/chromium/tools/depot_tools/+/6d3c290164360907fe08784093e36314069e689f

TBR=hinoka@chromium.org,maruel@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=635641

Review-Url: https://codereview.chromium.org/2295043003