[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