[Cashtab] [Deprecate BCH msg signing p3/3] Deprecate msg signing with bchObj

Summary:
T2730

Depends on D12627

Replace legacy signing method with updated local method. Since BCH param is now unused, it must be removed in this diff from this component and snapshot tests to prevent build errors.

Test Plan: npm start, sign and verify some msgs, confirm that ElectrumABC is also able to verify the msgs

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12628