netfilter: kill nf_send_reset6() from include/net/netfilter/ipv6/nf_reject.h

nf_send_reset6() now resides in net/ipv6/netfilter/nf_reject_ipv6.c

Fixes: c8d7b98b ("netfilter: move nf_send_resetX() code to nf_reject_ipvX modules")
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Eric Dumazet <edumazet@google.com>