[Cashtab] Remove all unused imports in chronik.test.js

Summary: Related to T2713. Unused BigNumber and currency imports have been removed from chronik.test.js

Test Plan:
navigate to web/cashtab/src/utils/__tests__/chronik.test.js
observe that the 'BigNumber' import is no longer present on line 2
observe that the 'currency' import is no longer present on line 1

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12410