[git-cache] Fix remove files recursively

os.remove can't be used to remove non empty directory. Use rmtree from
gclient_utils.

R=jojwang@google.com

Bug: 1372658
Change-Id: I5c08bad9778396cb9b1ec0bebe6287a0f3823fab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3950469
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Joanna Wang <jojwang@chromium.org>
Auto-Submit: Josip Sokcevic <sokcevic@google.com>

git_cache.py

@@ -521,7 +521,7 @@ class Mirror(object):
                                 self.mirror_path.split(os.path.sep)[-1])
     if os.path.exists(recursed_dir):
       self.print('Deleting unexpected directory: %s' % recursed_dir)
-      os.remove(recursed_dir)
+      gclient_utils.rmtree(recursed_dir)
 
     # The folder is <git number>
     gen_number = subprocess.check_output(