gnrc: remove legacy neighbor discovery code

Makefile.dep

examples/gnrc_minimal/Makefile

sys/include/net/gnrc.h

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

sys/include/net/gnrc/ndp.h

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

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

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

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

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

sys/net/gnrc/Makefile

sys/net/gnrc/network_layer/icmpv6/gnrc_icmpv6.c

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

sys/net/gnrc/network_layer/ipv6/nc/Makefile

sys/net/gnrc/network_layer/ipv6/nc/gnrc_ipv6_nc.c

sys/net/gnrc/network_layer/ndp/Makefile

sys/net/gnrc/network_layer/ndp/gnrc_ndp.c

sys/net/gnrc/network_layer/ndp/host/Makefile

sys/net/gnrc/network_layer/ndp/host/gnrc_ndp_host.c

sys/net/gnrc/network_layer/ndp/internal/Makefile