xbee: remove NETOPT_IPV6_IID support
xbee: remove NETOPT_IPV6_IID support

This option is handled in the `gnrc_netif`-layer without any access to
the driver's option since 7ae90564d91284dd5be63e89e4be1b4c6e8cfdef.

RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,kYc0o/RIOT,jasonatran/RIOT,kYc0o/RIOT,kaspar030/RIOT,kaspar030/RIOT,OTAkeys/RIOT,authmillenon/RIOT,authmillenon/RIOT,OTAkeys/RIOT,miri64/RIOT,authmillenon/RIOT,kaspar030/RIOT,OTAkeys/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,miri64/RIOT,kYc0o/RIOT,jasonatran/RIOT,ant9000/RIOT,authmillenon/RIOT,ant9000/RIOT,jasonatran/RIOT,OlegHahm/RIOT,jasonatran/RIOT,jasonatran/RIOT,OTAkeys/RIOT,miri64/RIOT,miri64/RIOT,OTAkeys/RIOT,ant9000/RIOT,ant9000/RIOT,OlegHahm/RIOT,miri64/RIOT,authmillenon/RIOT,kYc0o/RIOT,authmillenon/RIOT,kaspar030/RIOT