Fix empty directory removal for log expiration

* modules/openstack_project/files/log_archive_maintenance.sh: The
refactoring in I5ff6defb4d403f3017ab098c2ac1df3777662933 orphaned a
stray -not in the directory removal expression, inverting the
meaning of -empty as a result. Luckily non-empty directories can't
be removed without additional effort, so no data was lost (just some
very large cronspam sent instead). This removes the remaining cruft.

Change-Id: Ie6ce39ed46eedfaaf0771ab1a91f5f50c90c1b62
Merge "Fix empty directory removal for log expiration"