Remove Python from the command in CheckGNFormatted

The command is passed directly to gn.main, so it does not
need the python executable.

Change-Id: I010bf68e86bf48b4b7387c26124ede216696449f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6793062
Reviewed-by: Brian Ryner <bryner@google.com>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>

presubmit_canned_checks.py

@@ -1878,7 +1878,6 @@ def CheckGNFormatted(input_api, output_api):
     warnings = []
     for f in affected_files:
         cmd = [
-            input_api.python3_executable,
             input_api.os_path.join(_HERE, 'gn.py'), 'format', '--dry-run',
             f.AbsoluteLocalPath()
         ]