[NETFILTER]: nf_conntrack: change nf_conntrack_l[34]proto_unregister to void

No caller checks the return value, and since its usually called within the
module unload path there's nothing a module could do about errors anyway,
so BUG on invalid conditions and return void.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>