Fixed #18687: Removed test_performance_scalability

Even after repeated adjustment of the constants, this test still fails
randomly. It has educated us to ignore messages from Jenkins, to a
point where we missed some actual failures. In addition, it accounts
for a non-negligible percentage of the run time of the test suite
just by itself. Since no one has proposed a convincing patch in months,
I'm going to remove the patch. We can't keep a randomly failing test
forever.