netfilter: nf_conntrack_ipv6: delete the redundant macro definitions

The following three macro definitions are never used, so delete them.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Patrick McHardy <kaber@trash.net>