[Cashtab] Deprecate BCH param from generateTxOutput function

Summary:
T2730

Depends on D12664

Deprecate BCH param from generateTxOutput function. The param is there only as an artifact and is not used in the function.

Test Plan:
npm test
grep -r generatTxOutput src/ and make sure the param is removed from all instances of the function
npm start and send an XEC tx

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D12665