gnrc: remove legacy network interface structures

examples/gnrc_minimal/Makefile

sys/auto_init/auto_init.c

sys/include/net/gnrc/ipv6/nc.h

sys/include/net/gnrc/ipv6/netif.h

sys/include/net/gnrc/ndp.h

sys/include/net/gnrc/ndp/host.h

sys/include/net/gnrc/ndp/router.h

sys/include/net/gnrc/netif.h

sys/include/net/gnrc/rpl/structs.h

sys/include/net/gnrc/sixlowpan.h

sys/include/net/gnrc/sixlowpan/nd/router.h

sys/include/net/gnrc/sixlowpan/netif.h

sys/net/gnrc/Makefile

sys/net/gnrc/netif/Makefile

sys/net/gnrc/netif/gnrc_netif.c

sys/net/gnrc/netif/gnrc_netif_addr_from_str.c

sys/net/gnrc/netif/gnrc_netif_addr_to_str.c

sys/net/gnrc/nettest/gnrc_nettest.c

sys/net/gnrc/network_layer/icmpv6/error/gnrc_icmpv6_error.c

sys/net/gnrc/network_layer/ipv6/gnrc_ipv6.c