[refactor] replace calls to Params() CWallet chainParams member

Summary: as suggested in D7041, in the future this will make it easier to grab these params from CWallet's interface::chain() instead

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7736