This reverts commit f059ec93.

Reason for revert: Reverting because we probably need to be using vpython and a compiled crcmod instead. See, e.g.,. b/188591640.

Original change's description:
> Update gsutil to use gsutil version 4.61, python3.
>
> This CL updates the gsutil.py wrapper to download and use
> v4.61 of GCP's gsutil, which is Python3-compatible.
>
> v4.61 appears to be fully self-contained and have all of the
> packages it needs vendored into it. So, there's no reason to
> use vpython anymore, and this CL removes that.
>
> Also, this CL removes the 'fallback' option to gsutil and
> the ability to force a version switch, as this should no
> longer be necessary (it was added for a migration back in 2014
> but apparently this code was never removed afterwards).
>
> This CL also updates download_from_google_storage.py and
> upload_to_google_storage.py to similarly not have the version flags
> and to just use regular python3, not vpython3.
>
> Bug: 1184108
> Change-Id: I0d1a8351dba2d3ad1f927afa333fb10959f19443
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2898439
> Reviewed-by: Mike Frysinger <vapier@chromium.org>
> Reviewed-by: Josip Sokcevic <sokcevic@google.com>
> Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
> Commit-Queue: Dirk Pranke <dpranke@google.com>

Bug: 1184108
Change-Id: I8e21a9a40d81e4e185642f866855b6838f80f1c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2905904
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>