unused `classes` argument in `Dhcpv6Srv::classifyByVendor`

There's a string that was historically used for logging, but since 2016 (5db1edfb), it's not anymore. Code from 2016:

        LOG_DEBUG(dhcp6_logger, DBG_DHCP6_BASIC, DHCP6_CLASS_ASSIGNED)
            .arg(classes);

I suggest we remove the classes argument.

The no-op statement that involves it (classes + VENDOR_CLASS_PREFIX + vclass->getTuple(0).getText() + " ";) makes you think that custom vendor options don't lead to classification and it made me waste some time looking into whether it's a bug or not, while looking at issue 2517.