Remove unused test-metadata code

What does this MR do and why?

Follow-up of #377423 (closed) now that the experiment has proven to be successful!

TODO:

• Remove the RETRIEVE_TESTS_METADATA_FROM_PAGES variable from this project's CI/CD settings after a few months for back-compatibility (I don't expect any breakage as the old method would still work in old branches, but it's less risky to keep it enabled via the variables for these cases).

MR acceptance checklist

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