Even if there's no zebra server zebraqueue table is populated (with done == 0). misc/cronjobs/cleanup_database.pl is only deleting zebraqueue table with done == 1 But without a zebra indexer daemon nothing is setting zebraqueue.done to 1.
I think this could be resolved with bug 21820 - marking as a duplicate. *** This bug has been marked as a duplicate of bug 21820 ***