[Cashtab] Deprecate BCH param from generateTxInput function

Summary:
T2730

Remove BCH param from generateTxInput function

The param is not currently used in the function.

Test Plan:
npm start and send an XEC tx
npm test
grep -r generateTxInput src/ and make sure all instances have been addressed

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12663