[Cashtab] Patch use of legacy const missed in rebase

Summary: The diff that implemented this function was written before consolidating Cashtab's dust constants but landed after. Missed updating the constants here. Patch.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15990