delete routers with expired SSU2 introducers

libi2pd/NetDb.cpp

@@ -628,7 +628,8 @@ namespace data
 				(it.second->IsFloodfill () && totalFloodfills - deletedFloodfillsCount < NETDB_MIN_FLOODFILLS)))
 				it.second->SetUnreachable (false);
 			// find & mark expired routers
-			if (!it.second->IsReachable () && it.second->IsSSU (false))
+			if (!it.second->IsReachable () && (it.second->GetCompatibleTransports (true) & (RouterInfo::eSSUV4 | RouterInfo::eSSU2V4)))
+			// non-reachable router, but reachable by ipv4 SSU or SSU2 means introducers	
 			{
 				if (ts > it.second->GetTimestamp () + NETDB_INTRODUCEE_EXPIRATION_TIMEOUT*1000LL)
 				// RouterInfo expires after 1 hour if uses introducer