[NETFILTER]: nf_conntrack: remove unused struct list_head from protocols

Remove unused struct list_head from struct nf_conntrack_l3proto and
nf_conntrack_l4proto as all protocols are kept in arrays, not linked
lists.

Signed-off-by: Martin Josefsson <gandalf@wlug.westbo.se>
Signed-off-by: Patrick McHardy <kaber@trash.net>