This reverts commit e9013eeb.

Reason for revert: https://crbug.com/1064547

Original change's description:
> Reland "depot_tools: Run using Python 3 by default."
> 
> This is a reland of 01ed3589
> 
> * python_runner.sh, gclient, roll-dep and fetch will call vpython
>   (instead of vpython3) when running on Windows under git-bash.
> * vpython3 now detects when running on Windows under git-bash and
>   executes vpython3.bat instead.
> * vpython3.bat calls python3.exe directly instead of calling python3.bat.
> 
> Original change's description:
> > depot_tools: Run using Python 3 by default.
> >
> > Run gclient, roll-dep, fetch and custom git commands (i.e. git-cl, git-rebase-update, git-new-branch, etc.)
> > using vpython3 by default.
> >
> > Change-Id: I4eecddafa6ca4c5f82ec097615c79d2a741613e7
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2113550
> > Reviewed-by: Anthony Polito <apolito@google.com>
> > Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
> 
> Change-Id: I9829141d7ea26a67e655264430151f493e73a930
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2118418
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
> Reviewed-by: Anthony Polito <apolito@google.com>

TBR=ehmaldonado@chromium.org,ajp@chromium.org,apolito@google.com,infra-scoped@luci-project-accounts.iam.gserviceaccount.com,sokcevic@google.com

Change-Id: Ia382d6e4e330f5a7a7a62e4d369c3fa16fc17333
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2120956Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>