[backport#15784] rpc: Remove dependency on interfaces::Chain in SignTransaction

Summary:
Comment SignTransaction utility

https://github.com/bitcoin/bitcoin/pull/15784/commits/99e88a3726c2325e3a3a35c0a750bde25bd58ad0

Depends on D6257

Backport of Core PR15784

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6258