Revert "Use bundled python to run gn"

This reverts commit fa8fc854e1766b86f10c9a15902cf3cc23adaac2.

Reason for revert: Most likley culprit for consistent failure
to generate orderfiles (see crbug.com/414312048).

Bug: 414312048

Original change's description:
> Use bundled python to run gn
>
> Change-Id: If01a7dae1aca72c93c87c1db27d2d37432094852
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6490515
> Reviewed-by: Junji Watanabe <jwata@google.com>
> Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
> Auto-Submit: Fumitoshi Ukai <ukai@google.com>
> Commit-Queue: Fumitoshi Ukai <ukai@google.com>

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: Id5e5f7cec91927aae144026aa16b169689881fb2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6494846
Owners-Override: Friedrich Hauser <friedrichh@chromium.org>
Auto-Submit: Friedrich Hauser <friedrichh@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>

gn

@@ -11,4 +11,4 @@ if [ "$(expr "$(uname -s)" : "^MINGW64_NT")" == "10" ]; then
     exit
 fi
 
-PYTHONDONTWRITEBYTECODE=1 "$base_dir/python-bin/python3" "$base_dir/gn.py" "$@"
+PYTHONDONTWRITEBYTECODE=1 exec python3 "$base_dir/gn.py" "$@"

gn.bat

@@ -8,7 +8,5 @@ setlocal
 :: standalone, but allow other PATH manipulations to take priority.
 set PATH=%PATH%;%~dp0
 
-set scriptdir=%~dp0
-
 :: Defer control.
-@call %scriptdir%python-bin\python3 "%~dp0\gn.py" %*
+python3 "%~dp0\gn.py" %*