[Cashtab] Better code comments in parseChronikTx

Summary:
T2853

Remove artifact comments related to now-defunct useBCH.js

Test Plan:
Confirm this only impacts comments
npm start
tx history parses
Send an encrypted msg
Receive an encrypted msg

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D12839