Don't perform unilateral close for minimal-value HTLCs

[rustyrussell]

@fiatjaf pointed out that this is not economically sane. There are still issues with closing out an HTLC where the peer may come back and claim it with the preimage, but you're no worse off since you can drop to chain (or just eat the loss?) at that point.

Dropping to chain does not allow you to collect dust HTLCs anyway, it's just a punishment mechanism. He also suggested we could use an arbiter for this case, but that's future work.