health: Remove the fake petiole policy used for testing once round robin delegates to pickfirst

[arjan-bal]

As part of #7900, an alternate code path was added in the health check tests to use the new pickfirst with health listeners. This is required to have the existing health check tests run using the health producer, ensuring backward compatibility and providing test coverage for the changes in the PR. Once round robin delegates to pickfirst, we can revert the changes for adding the fake petiole policy and only test round robin.