[Cashtab] Deprecate legacy transaction functions

Summary:
These functions have been replaced by implemented functions in transactions/index.js

The remaining functions in utils/transactions.js are still pending some additional SLP helper functions

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15068