ipv6: ndp: sixlowpan: remove vtimer includes

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

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

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

sys/net/gnrc/network_layer/ndp/internal/gnrc_ndp_internal.c

sys/net/gnrc/network_layer/ndp/router/gnrc_ndp_router.c

...net/gnrc/network_layer/sixlowpan/ctx/gnrc_sixlowpan_ctx.c

sys/net/gnrc/network_layer/sixlowpan/frag/rbuf.c