Revert "Add Python 3 support to GetPythonUnitTests"

This reverts commit dfc71bbe.

Reason for revert: Breaks all presubmits that do not have skip_shebang_check=True set.

Original change's description:
> Add Python 3 support to GetPythonUnitTests
>
> GetPythonUnitTests (and by extension RunPythonUnitTests) always ran
> scripts using Python 2. This change adds a python3=False parameter to
> GetPythonUnitTests to give the option of switching to Python 3, while
> leaving the default as Python 2. The child scripts are run under one or
> the other, based on this parameter.
>
> This change is needed in support of crrev.com/c/3679801.
>
> Bug: 1313804
> Change-Id: Ic59287352d4941707adaf7981ed7af4201b7d526
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3680099
> Reviewed-by: Jesse McKenna <jessemckenna@google.com>
> Commit-Queue: Bruce Dawson <brucedawson@chromium.org>

Bug: 1313804
Change-Id: I1bd7096b6cfdfed6205e6dc0b5d4498b5b821b97
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3683378
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Christoffer Jansson <jansson@chromium.org>
Owners-Override: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Tomas Gunnarsson <tommi@chromium.org>

presubmit_canned_checks.py

@@ -882,7 +882,7 @@ def GetUnitTestsRecursively(input_api,
                       skip_shebang_check=skip_shebang_check)
 
 
-def GetPythonUnitTests(input_api, output_api, unit_tests, python3=False):
+def GetPythonUnitTests(input_api, output_api, unit_tests):
   """Run the unit tests out of process, capture the output and use the result
   code to determine success.
 
@@ -921,10 +921,7 @@ def GetPythonUnitTests(input_api, output_api, unit_tests, python3=False):
         backpath.append(env.get('PYTHONPATH'))
       env['PYTHONPATH'] = input_api.os_path.pathsep.join((backpath))
       env.pop('VPYTHON_CLEAR_PYTHONPATH', None)
-    if python3:
-      cmd = [input_api.python3_executable, '-m', '%s' % unit_test]
-    else:
-      cmd = [input_api.python_executable, '-m', '%s' % unit_test]
+    cmd = [input_api.python_executable, '-m', '%s' % unit_test]
     results.append(input_api.Command(
         name=unit_test_name,
         cmd=cmd,