Remove git drover

git drover was deprecated 3 years ago in favor of cherry picking:
https://crrev.com/c/2653952

Change-Id: I12273f7691de73eef939a13ac5a4fdfec65356f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5153632
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>

.vpython

@@ -52,7 +52,6 @@ wheel: <
 #   tests/gclient_utils_test.py
 #   tests/gerrit_util_test.py
 #   tests/git_cl_test.py
-#   tests/git_drover_test.py
 #   tests/git_footers_test.py
 #   tests/metrics_test.py
 #   tests/presubmit_unittest.py

git_drover.py

@@ -1,47 +0,0 @@
-#!/usr/bin/env python3
-# Copyright 2015 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.
-
-import argparse
-
-_HELP_MESSAGE = """\
-git drover has been deprecated in favor of cherry-picking using Gerrit.
-Try it, it's faster!
-
-See https://www.chromium.org/developers/how-tos/drover for instructions.
-
-If the Gerrit UI is not sufficient, and you know what you're doing:
-  git checkout -b branch-name refs/remotes/branch-heads/{branch}
-  git cherry-pick -x {cherry_pick}
-
-If you have to do a lot of merges, consider using multiple working directories
-in your checkout:
-https://www.chromium.org/developers/how-tos/get-the-code/multiple-working-directories
-"""
-
-
-def main():
-    parser = argparse.ArgumentParser(description=_HELP_MESSAGE)
-    parser.add_argument(
-        '--branch',
-        default='BRANCH',
-        metavar='BRANCH',
-        type=str,
-        help='the name of the branch to which to cherry-pick; e.g. 1234')
-    parser.add_argument(
-        '--cherry-pick',
-        default='HASH_OF_THE_COMMIT_TO_CHERRY_PICK',
-        metavar='HASH_OF_THE_COMMIT_TO_CHERRY_PICK',
-        type=str,
-        help=('the change to cherry-pick; this can be any string '
-              'that unambiguosly refers to a revision not involving HEAD'))
-    options, _ = parser.parse_known_args()
-
-    print(
-        _HELP_MESSAGE.format(branch=options.branch,
-                             cherry_pick=options.cherry_pick))
-
-
-if __name__ == '__main__':
-    main()