[Cashtab] remove unused imports and functions from Wallet.js

Summary:
T1733

Remove imports and functions from Wallet.js that are no longer used. These were missed in earlier diff that removed support for bitcoincash: prefixes.

Test Plan: npm test

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9889