[Chronik] Fix chronik_mempool_disconnectpool.py

Summary: This was errorneously merged previously and the CI didn't catch it.

Test Plan: ./test/functional/test_runner.py chronik_mempool_disconnectpool

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16646