[ecashjs-message] Fix unit tests

Summary:
T3084

Remove segwit unit tests so that npm test passes

Background: This repo was added to support message signing and verification with ecash: addresses and ecash: signing prefix. I also deprecated segwit from the repo since this complicated things. However, segwit was not deprecated from the unit tests.

This lib still needs some work to get up to monorepo standards, i.e. ecash: unit tests need to be added.

Test Plan: npm test all pass

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13579