[Cashtab] [ECPair from wif p2/3] Deprecate BCH method

Summary:
T2730

Depends on D12616

Remove BCH param from function and migrate to new local method

Test Plan:
npm test and confirm existing unit tests of function that depends on this param still work
npm start, send some transactions, confirm they work

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12618