Revert "test: check custom descendant limit in mempool_packages.py"

Summary:
This reverts D8770

The test added in D8770 causes intermittent but frequent failures on various CI builds. I still haven't been able to reproduce the failure on my workstation, making the debugging difficult.

Test Plan: test/functional/test_runner.py mempool_packages

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8851