Storage leak: `used_from_tx` never cleaned up

[wmagev]

used_from_tx (smart-contracts/ink/lib.rs:42) is inserted into at smart-contracts/ink/lib.rs:711 but never removed — neither confirm_swap (:805) nor timeout_swap (:862) touch it.

Every initiated swap leaves one String entry in contract storage permanently.

Same class of bug as the request_votes leak already under discussion — should be fixed together.