netfilter: nf_conntrack: remove unnecessary RTNL locking

Locking the rtnl was added to nf_conntrack_l{3,4}_proto_unregister()
for walking the network namespace list. This is not done anymore since
we have proper namespace support in the protocols now, so we don't
need to take the RTNL anymore.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>