remove publish handlers on disconnect seems not making sense if qos > 0

[liaojinxing]

static void on_disconnect(struct mosquitto *mosq, void *obj, int rc)
{
    MQTTClient* client = (__bridge MQTTClient*)obj;
    LogDebug(@"[%@] on_disconnect rc = %d", client.clientID, rc);
    [client.publishHandlers removeAllObjects];
    [client.subscriptionHandlers removeAllObjects];
    [client.unsubscriptionHandlers removeAllObjects];
    client.connected = NO;
    if (client.disconnectionHandler) {
        client.disconnectionHandler(rc);
    }
}

Inside on_disconnect method, remove all the callbacks including publishHandlers, subscriptionHandlers, and unsubscriptionHandlers.
If the publish qos is AtLeastOnce or ExactlyOnce, remove the publishHandlers on disconnect seems not making sense.