zone: flush event properly rescheduled after each flush

Trying to get rid of that TODO...

src/knot/events/handlers/flush.c

@@ -34,14 +34,5 @@ int event_flush(conf_t *conf, zone_t *zone)
 		return ret;
 	}
 
-	zone->timers.last_flush = time(NULL);
-
-	conf_val_t val = conf_zone_get(conf, C_ZONEFILE_SYNC, zone->name);
-	int64_t sync_timeout = conf_int(&val);
-	if (sync_timeout > 0) {
-		time_t next_flush = zone->timers.last_flush + sync_timeout;
-		zone_events_schedule_at(zone, ZONE_EVENT_FLUSH, next_flush);
-	}
-
 	return KNOT_EOK;
 }