Send NOFITY messages after deleting private-type records.

The `rndc signing -clear` command cleans up the private-type records
that keep track of zone signing activity, but before this change it
did not tell the secondary servers that the zone has changed.

lib/dns/zone.c

@@ -19576,7 +19576,8 @@ keydone(isc_task_t *task, isc_event_t *event) {
 		commit = true;
 
 		LOCK_ZONE(zone);
-		DNS_ZONE_SETFLAG(zone, DNS_ZONEFLG_LOADED);
+		DNS_ZONE_SETFLAG(zone, DNS_ZONEFLG_LOADED|
+				 DNS_ZONEFLG_NEEDNOTIFY);
 		zone_needdump(zone, 30);
 		UNLOCK_ZONE(zone);
 	}