Removing last node to join.

Currently, it is up to the last node to join whether to leave.
If you allow any member of the swap to remove the last member to join, it would possibly help swap participants to faster resolve when a rule-breaker joins such as https://lightningnetwork.plus/swaps/8488

Please destroy https://lightningnetwork.plus/swaps/8488 if the rulebreaker there can't be removed.