Reland "Drop support for py2 presubmit hook win mapping"

This is a reland of commit e27d7c3816

Original change's description:
> Drop support for py2 presubmit hook win mapping
>
> We no longer support py2 so we shouldn't have a special handling for py2
> hooks.
>
> R=gavinmak@google.com
>
> Change-Id: I038f462a4991d17bedb3e02104374b3ad1ffa5ff
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4541664
> Commit-Queue: Gavin Mak <gavinmak@google.com>
> Reviewed-by: Gavin Mak <gavinmak@google.com>
> Auto-Submit: Josip Sokcevic <sokcevic@chromium.org>

Change-Id: I190fb7c1ee4e76adfe74196c1ed579a424314a02
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4669591
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>

gclient.py

@@ -247,9 +247,7 @@ class Hook(object):
 
     cmd = list(self._action)
 
-    if cmd[0] == 'python':
-      cmd[0] = 'vpython'
-    if (cmd[0] in ['vpython', 'vpython3']) and _detect_host_os() == 'win':
+    if cmd[0] == 'vpython3' and _detect_host_os() == 'win':
       cmd[0] += '.bat'
 
     exit_code = 2