Remove stability and benchmark metric infrastructure

[brson]

We generate this information and upload it and do nothing with it (and the upload causes #19079). Let's just completely remove the code for it (including the never-used ratcheting feature, and probably the test-sharding feature as well).

[brson]

Nominating because this adds --save-metrics and --ratchet-metrics to test runners that I don't want to support.

[steveklabnik]

I left the documentation about saving and ratcheting metrics out of the latest iteration of the testing guide, so agreed there.

[brson]

Bots are no longer uploading metrics so it's safe to remove from the code.

[pnkfelix]

P-backcompat-libs, 1.0.