[Cashtab] Remove unused var from useBCH

Summary: Removed unused 'prefix' var from cashaddr obj in useBCH, related to task T2063.

Test Plan:
cd web/cashtab/src/hooks
inspect useBCH.js
ensure 'prefix' is removed from obj on line 134
test the reply to message function where the sender of a TX is displayed in the address input field

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10643