Revert "Update gsutil to use gsutil version 5.6, python3"

This reverts commit c0af61c8dd.

Reason for revert: blocked on https://crrev.com/c/3440919

Original change's description:
> Update gsutil to use gsutil version 5.6, python3
>
> R=​gavinmak@google.com
>
> Bug: 1184108
> Change-Id: I933c07132006b731a97153080ebe86ed756fbdb2
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3497998
> Reviewed-by: Gavin Mak <gavinmak@google.com>
> Commit-Queue: Josip Sokcevic <sokcevic@google.com>

Bug: 1184108
Change-Id: Ifd553d12fcc25230de5ba9e6c32c8e5190cf01a8
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3500910
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

gsutil.py.vpython3

@@ -1,19 +0,0 @@
-# vpython VirtualEnv protobuf for "gsutil".
-#
-# See:
-# https://chromium.googlesource.com/infra/luci/luci-go/+/HEAD/vpython/api/vpython/spec.proto
-#
-# This is a "vpython" VirtualEnv specification applied to invocations of "gsutil"
-# by the bootstrap wrapper, "gsutil.py". It ensures that any Python distribution
-# has the expected Python packages installed.
-#
-# This is specifically relevant on Google Compute Engine invocations of
-# "gsutil", where a stock system-deployed file in "/etc/" explicitly specifies
-# that the "google_compute_engine" and "boto" Python packages are available.
-
-python_version: "3.8"
-
-wheel: <
-  name: "infra/python/wheels/crcmod/${vpython_platform}"
-  version: "version:1.7.chromium.3"
->