[ecash-herald] Remove one-off promise building function from utils and add to its parent function

Summary:
T2972

Depends on D14099

Remove promise building function from utils. Include logic in parent function, only place where it is relevant and used.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14140