When there's a symlink to a directory, os.walk still lists the symlink
in dirs, even if it isn't configured to follow symlinks. This will fail
the listdirs check if the symlink is broken (either before or during the
cleanup). So instead, check for directory symlinks and remove them using
os.remove.
Bug: Issue 231
Change-Id: I0ec45a26be566613a4a39bf694a3d9c6328481c2