[herald] Remove un-needed .js from scripts in package.json

Summary:
T2972

Depends on D13618

Scripts in package.json do not need the .js. Remove.

Test Plan: npm run mocks works

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13619