[Cashtab] Remove unused imports from test file

Summary:
T2730

Remove unused imports from cashMethods.test.js. Some of these were missed in earlier clean-up diffs, others are lint errors.

Test Plan: npm test

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D12641