Cleanup phonon activation in functional tests

Summary:
Some phonon activation logic is left in abc-miner-fund to ensure
we do not break this logic. Trying to clean up that logic further would
end up scrapping large parts of that test.

Test Plan:

test_runner.py abc-miner-fund
test_runner.py mempool_packages

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6796