Remove much of the remaining BIP9 code

Summary:
Much of the BIP9 handling code was removed in the following revisions:

• Remove BIP9 softforks test https://reviews.bitcoinabc.org/D1940
• Remove bip9 from getblockchaininfo RPC call https://reviews.bitcoinabc.org/D1550
• Remove bip9 version bits warning https://reviews.bitcoinabc.org/D1539
• Remove the -bip9params flag https://reviews.bitcoinabc.org/D1538

This commit removes the unused versionbits handling code, and remaining extern references.

Test Plan:

make check && ./test/functional/test_runner.py

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D2783