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/+/2587576Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#836284}

.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.
 #