remove unused BIP9 code

Summary:
This is the part of the BIP9 code that is not used at all in Bitcoin ABC.

Following commits will remove more BIP9 code, but to do so some tests and RPC outputs will need to be removed.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13156