remove most of versionbits / BIP9 related code

Summary:
The only remaining piece is ComputeBlockVersion and code related to BuriedDeployments

The functional test mining_basic.py needs a change because the removed TESTDUMMY deployment was always active on testnet, and this is the only place where it was tested. There is no need to keep this test.

Depends on D13157

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13158