[Cashtab] Removed unused var from useWallet.js

Summary: Removed unused artifact var from useWallet.js. Related to task T2062

Test Plan:
cd web/cashtab/src/hooks
inspect useWallet.js
ensure unused previousWallet var is removed.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10639