Remove static definition of scenario classes

What does this MR do and why?

This removes static list of scenarios and fetches them automatically removing the need for manual maintenance of the list for selective execution.

Closes: #371847 (closed)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Test pipeline: https://gitlab.com/gitlab-org/gitlab/-/pipelines/664164113#/