[Cashtab] Deprecate legacy token checker function

Summary:
We have a better function available to check if the user has a certain quantity of a certain token. Use this instead of legacy method. Delete legacy method.

Add test to confirm expected behavior.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15595