gnrc_nettype: remove GNRC_NETTYPE_IOVEC
gnrc_nettype: remove GNRC_NETTYPE_IOVEC

NETTYPE is unused in the code base since
9fb2f541baca469e34fa01b004d6f19385700ce9 and deprecated since
744f078b3a506609476715c41460ed039f40620c

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