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.