[Cashtab] Remove unused imports from useWallet

Summary: Removed unused imports from useWallet.js, related to task: T2024.

Test Plan:
cd web/cashtab/src/hooks
open useWallet.js
ensure there are no unused imports

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10594