[Cashtab] Deprecate BCH param from getRecipientPublicKey function

Summary:
T2730

Depends on D12666

Remove BCH param from getRecipientPublicKey function. The param is not used and exists as an artifact.

Test Plan:
npm test
npm start and send an encrypted msg tx
grep -r getRecipientPublicKey src/ and ensure it's removed from all instances

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12667