Revert "my_activity.py: Run using vpython3 by default."

This reverts commit 488712ab57.

Reason for revert: Breaks when running on vpython3

Original change's description:
> my_activity.py: Run using vpython3 by default.
> 
> Also, prompt users to use vpython3 when running my_activity.py
> using Python 2.
> 
> Change-Id: I078517832520b3c7baf3324f83e9a3b5194a5bcb
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2079703
> Reviewed-by: Josip Sokcevic <sokcevic@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

TBR=ehmaldonado@chromium.org,apolito@google.com,infra-scoped@luci-project-accounts.iam.gserviceaccount.com,sokcevic@google.com

Change-Id: Ib217e8ec3613ee6645149f3d78ef9534ac68af8b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2080201
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

my_activity.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env vpython3
+#!/usr/bin/env vpython
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
@@ -46,8 +46,6 @@ import gerrit_util
 
 
 if sys.version_info.major == 2:
-  logging.warning(
-      'Python 2 is deprecated. Run my_activity.py using vpython3.')
   import urllib as urllib_parse
 else:
   import urllib.parse as urllib_parse