netfilter: remove nf_conntrack_icmpv6.h header.

nf_conntrack_icmpv6.h contains two object macros which duplicate macros
in linux/icmpv6.h.  The latter definitions are also visible wherever it
is included, so remove it.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>