It is a remnant of previsou unification of rmtree paths. Instead of
deleting tree directly while deleting repo it is put into queue now. So,
other thread looking for expired/deleted repos can find it also and add
it twice. Internal rmtree can check the queue before adding duplicate
path. As a side-effect manager.rmtree can also never fail, so try/except
can be removed from there.
Fixes: https://pagure.io/koji/issue/2716
e83dd2757e