protnum: remove ng_ prefix

sys/include/net/ng_ipv6.h

sys/include/net/ng_ipv6/ext.h

sys/include/net/ng_ipv6/hdr.h

sys/include/net/ng_nettype.h

sys/include/net/ng_protnum.h

sys/include/net/protnum.h

sys/net/network_layer/ng_icmpv6/ng_icmpv6.c

sys/net/network_layer/ng_ipv6/ext/ng_ipv6_ext.c

sys/net/network_layer/ng_ipv6/ext/rh/ng_ipv6_ext_rh.c

sys/net/network_layer/ng_ipv6/hdr/ng_ipv6_hdr.c

sys/net/network_layer/ng_ipv6/ng_ipv6.c

sys/net/transport_layer/ng_udp/ng_udp.c

tests/unittests/tests-ipv6_hdr/tests-ipv6_hdr.c