Remove inexistant parameter keypoolmin from integration test

Summary: Grepping for it, it never existed outside of the test framework in Core.

Test Plan:

test/functional/test_runner.py wallet_keypool_topup.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3715