kojira: Do not ever clean up repositories where 'latest' points to
We don't want to leave symlink broken… Ideally we would find out stuff
where we should even remove "latest" but it is not straightforward and I
believe this simple check is good enough for the time being.
Fixes: https://pagure.io/koji/issue/2832
Signed-off-by: Igor Raits <igor.raits@gmail.com>