Revert "Use depot_tools python3 to call ninja.py"

This reverts commit 8e77bba1ae.

Reason for revert: Very likely caused https://crbug.com/1399811

Original change's description:
> Use depot_tools python3 to call ninja.py
>
> This guarantees the python version used to call ninja.py.
>
> Bug: 1398687
> Change-Id: Ife58b285bb621dddb20b947010bc19818cd41395
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4091206
> Commit-Queue: Junji Watanabe <jwata@google.com>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Auto-Submit: Junji Watanabe <jwata@google.com>
> Commit-Queue: Nico Weber <thakis@chromium.org>

Bug: 1398687
Change-Id: I9bf469cf19134f58b3bbb44d4efa1959fe37b93f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4092694
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>

ninja

@@ -5,4 +5,4 @@
 # found in the LICENSE file.
 
 base_dir=$(dirname "$0")
-PYTHONDONTWRITEBYTECODE=1 exec "$base_dir/python-bin/python3" "$base_dir/ninja.py" "$@"
+PYTHONDONTWRITEBYTECODE=1 exec python3 "$base_dir/ninja.py" "$@"

ninja.bat

@@ -4,11 +4,9 @@
 :: found in the LICENSE file.
 setlocal
 
-set scriptdir=%~dp0
-
 :: Ensure that "depot_tools" is somewhere in PATH so this tool can be used
 :: standalone, but allow other PATH manipulations to take priority.
-set PATH=%PATH%;%scriptdir%
+set PATH=%PATH%;%~dp0
 
 :: Defer control.
-call %scriptdir%python-bin\python3.bat "%~dp0\ninja.py" %*
+python3 "%~dp0\ninja.py" %*