Revert "Call python3.bat explicitly from gn.bat on Windows."

This reverts commit eb28118b.

Reason for revert: This broke one of the v8 builders: https://ci.chromium.org/ui/p/devtools-frontend/builders/try/dtf_presubmit_win64/b8835470412847208097/overview, because it assumes that `python3.bat` is in $PATH. We need to change the CL to reference the python3.bat via ~dp0 instead, I think.

Original change's description:
> Call python3.bat explicitly from gn.bat on Windows.
>
> This ensures users running from a linux-style shell call through to
> the right Python interpreter.
>
> Bug: None
> Change-Id: I2e5f2d1a02537ee86f682607670199d37225cf15
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3168710
> Auto-Submit: Jamie Madill <jmadill@chromium.org>
> Commit-Queue: Dirk Pranke <dpranke@google.com>
> Reviewed-by: Dirk Pranke <dpranke@google.com>

Bug: None
Change-Id: I834e828c99e056887e0a84805c4c7a9e67220cfb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3173633
Auto-Submit: Dirk Pranke <dpranke@google.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>

gn.bat

@@ -9,4 +9,4 @@ setlocal
 set PATH=%PATH%;%~dp0
 
 :: Defer control.
-python3.bat "%~dp0\gn.py" %*
+python3 "%~dp0\gn.py" %*