Remove BIP9 dead code in util.py

Summary: No longer used in any place in the test framework nor tests.

Test Plan:

./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix, markblundeberg

Reviewed By: #bitcoin_abc, deadalnix, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3092