Revert "Reland "Force Python 3 to be used in build.""

This reverts commit 6ea5c38d.

Reason for revert: build failure: https://ci.chromium.org/ui/p/chromium/builders/ci/Deterministic%20Linux/30058/overview

Original change's description:
> Reland "Force Python 3 to be used in build."
>
> This reverts commit b6172196.
>
> Reason for revert: We rolled GN to a new version with a fix for
> the win dev issues, so hopefully is good to go now.
>
> Original change's description:
> > Revert "Force Python 3 to be used in build."
> >
> > This reverts commit bb97ab9b.
> >
> > Reason for revert: Various breakages.
> >
> > Original change's description:
> > > Force Python 3 to be used in build.
> > >
> > > This CL makes Python 3 be the default script executable used by Ninja
> > > to run action() and action_foreach()'s during the build.
> > >
> > > This does *not* mean that everything works with Python3; many targets
> > > may still use Python 2 via the python2_action() and
> > > python2_action_foreach() targets or via custom logic.
> > >
> > > However, it does mean that it should prevent regressions from
> > > creeping in.
> > >
> > > This CL should not be taken to imply that we can start to write
> > > Python3-specific code that can run during the build; we should
> > > still be trying to ensure that everything is compatible with Python2.
> > >
> > > Also, this CL does not enable the use of vpython in actions; we should
> > > still avoid that because vpython is slower than direct Python
> > > invocations.
> > >
> > > Bug: 1112471
> > > Change-Id: I472b032b500715156d955b0758640a7ed0384305
> > > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2515787
> > > Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
> > > Commit-Queue: Dirk Pranke <dpranke@google.com>
> > > Cr-Commit-Position: refs/heads/master@{#836234}
> >
> > TBR=dpranke@google.com,brucedawson@chromium.org,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com
> >
> > Change-Id: I72039140f401f1fefc1b515dcfe34f1984b0d914
> > No-Presubmit: true
> > No-Tree-Checks: true
> > No-Try: true
> > Bug: 1112471
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587576
> > Reviewed-by: Dirk Pranke <dpranke@google.com>
> > Commit-Queue: Dirk Pranke <dpranke@google.com>
> > Cr-Commit-Position: refs/heads/master@{#836284}
>
> TBR=dpranke@google.com,brucedawson@chromium.org,tikuta@chromium.org,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com
>
> # Not skipping CQ checks because original CL landed > 1 day ago.
>
> Bug: 1112471
> Change-Id: I55ee8ccb381b5aeeac58a85a6168765ed40457ba
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2630725
> Reviewed-by: Dirk Pranke <dpranke@google.com>
> Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
> Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
> Commit-Queue: Dirk Pranke <dpranke@google.com>
> Cr-Commit-Position: refs/heads/master@{#844885}

TBR=dpranke@google.com,brucedawson@chromium.org,tikuta@chromium.org,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com

Change-Id: I01ac20d80f5fe38a1f8113010f803509df0d7861
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1112471
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2637169Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844986}

.gn

@@ -8,10 +8,6 @@ import("//third_party/angle/dotfile_settings.gni")
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
-# The python interpreter to use by default. On Windows, this will look
-# for python3.exe and python3.bat.
-script_executable = "python3"
-
 # These arguments override the default values for items in a declare_args
 # block. "gn args" in turn can override these.
 #