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These functions have been replaced by implemented functions in transactions/index.js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