Revert "bot_update: remove cleanup_dir"

This reverts commit 4cebc13a4e.

Reason for revert: this breaks Dart's builds
https://ci.chromium.org/ui/p/dart/builders/ci.sandbox/dart2js-unit-linux-x64-release/27481/infra

Original change's description:
> bot_update: remove cleanup_dir
>
> when git cache is removed, gclient sync fails
> and old checkout is moved into cleanup_dir.
>
> It will cause out of disk error during build
> because cleanup_dir keeps full old checkout and build artifacts,
> which is no longer needed when bot_update
> succeeded to checkout.
>
> Remove cleanup_dir after checkout.
>
> Bug: 388307198
> Change-Id: I3858e718622a1ef2012fe5b80a5d9b128d96d1d4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6155955
> Commit-Queue: Jie Sheng <jiesheng@google.com>
> Reviewed-by: Yiwei Zhang <yiwzhang@google.com>
> Auto-Submit: Fumitoshi Ukai <ukai@google.com>
> Owners-Override: Jie Sheng <jiesheng@google.com>
> Reviewed-by: Junji Watanabe <jwata@google.com>
> Reviewed-by: Jie Sheng <jiesheng@google.com>

Bug: 388307198
Change-Id: Ifb68c1aaf28e2203ba2c3b5804c2fd4939bbd921
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6159604
Auto-Submit: Alexander Thomas <athom@google.com>
Reviewed-by: Jie Sheng <jiesheng@google.com>
Commit-Queue: Yiwei Zhang <yiwzhang@google.com>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -16,7 +16,6 @@ import optparse
 import os
 import pprint
 import re
-import shutil
 import subprocess
 import sys
 import tempfile
@@ -1143,16 +1142,7 @@ def checkout(options, git_slns, specs, revisions, step_text):
   properties['git_cache_epoch'] = cache_epoch
 
   usage = disk_usage()
-  # successfully checked out. remove cleanup_dir to get free disk space.
-  if os.path.exists(cleanup_dir):
-    prev_usage = usage
-    print('Removing cleanup_dir %s' % cleanup_dir)
-    shutil.rmtree(cleanup_dir, ignore_errors=True)
-    usage = disk_usage()
-    print('Release %dGB (%d%%)' %
-          (prev_usage.used_disk_space_gb - usage.used_disk_space_gb,
-           prev_usage.percent_used - usage.percent_used))
-
+  # TODO(ukai): remove cleanup_dir if available disk space is small?
   step_text = step_text + (' -> [%dGB/%dGB used (%d%%)]' % usage)
 
   # Tell recipes information such as root, got_revision, etc.