[Cashtab] Deprecate all duplicated legacy implementations of toXec and toSatoshis

Summary:
Bad buildup of functions that do basically the same thing. Get rid of it.

Depends on D15217

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15218