relay: Don't print ::ffff: in IPv4-mapped IPv6 addresses

I think this is the only place in WeeChat where this happens. In my opinion ::ffff: is an internal detail of how IPv4 addresses are dealt with in IPv6 socket structures, and shouldn't be printed anywhere. 

This also simplifies `relay.network.allowed_ips`, as the same IPv4 IP will be identical, regardless of whether it was handled by an IPv6 or IPv4 socket.