finish backporting PR9602 - remove unused and untested miner code
Summary:
https://github.com/bitcoin/bitcoin/pull/9602/files
unused after D4778
Test Plan: ninja all check
Reviewers: #bitcoin_abc, deadalnix, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D4810