iptables-nft: allow removal of empty builtin chains

iptables-nft: allow removal of empty builtin chains

The only reason why this is prohibited is that you cannot do it
in iptables-legacy.

This removes the artifical limitation.

"iptables-nft -X" will leave the builtin chains alone;
Also, deletion is only permitted if the chain is empty.

Signed-off-by: Florian Westphal <fw@strlen.de>