Revert "Switch download_from_google_storage scripts to invoke python3."

This reverts commit f6e3b7fb.

Reason for revert: See https://ci.chromium.org/ui/p/chromium/builders/try/linux-rel/665976/overview - looks like the bots don't have depot_tools populated :(.

Original change's description:
> Switch download_from_google_storage scripts to invoke python3.
>
> Bug: 941669
> Change-Id: I65a9ca3453858c447f6533d9a3a4eade4a825aa7
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2825441
> Reviewed-by: Dirk Pranke <dpranke@google.com>
> Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>

Bug: 941669
Change-Id: I69856a8c68394ac42126ac68b38d68a6c0104872
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2827161
Auto-Submit: Dirk Pranke <dpranke@google.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>

download_from_google_storage

@@ -5,4 +5,4 @@
 
 base_dir=$(dirname "$0")
 
-PYTHONDONTWRITEBYTECODE=1 exec $base_dir/python-bin/python3 "$base_dir/download_from_google_storage.py" "$@"
+PYTHONDONTWRITEBYTECODE=1 exec python "$base_dir/download_from_google_storage.py" "$@"

download_from_google_storage.bat

@@ -9,4 +9,4 @@ setlocal
 set PATH=%PATH%;%~dp0
 
 :: Defer control.
-%dp0\python-bin\python3.bat "%~dp0\download_from_google_storage.py" %*
+python "%~dp0\download_from_google_storage.py" %*

download_from_google_storage.py

-#!/usr/bin/env python3
+#!/usr/bin/env python
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.