[Cashtab] Deprecate BCH param from signAndBuildTx function

Summary:
T2730

Depends on D12665

Deprecate BCH param from signAndBuildTx function. The param is not used in the function and exists as an artifact.

Test Plan:
npm test
npm start and send an XEC tx
grep -r signAndBuildTx src/ and make sure it's removed everywhere

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12666