netfilter: conntrack: avoid use-after free on rmmod

When the conntrack module is removed, we call nf_ct_iterate_destroy via
nf_ct_l4proto_unregister().

Problem is that nf_conntrack_proto_fini() gets called after the
conntrack hash table has already been freed.

Just remove the l4proto unregister call, its unecessary as the
nf_ct_protos[] array gets free'd right after anyway.

v2: add comment wrt. missing unreg call.

Fixes: a0ae2562

 ("netfilter: conntrack: remove l3proto abstraction")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>