[Cashtab] Remove unused imports useBCH.test.js

Summary: Removed unused imports from useBCH.test.js, related to T2714.

Test Plan:
cd web/cashtab && npm start
navigate to cashtab/src/hooks/__tests__/useBCH.test.js
observe that there are no unused imports

npm test

all tests should pass

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12188