Stop override tracking in experiment stub [RUN ALL RSPEC]

What does this MR do?

This is removing the override of should_track?. It got introduced with !55072 (merged) as a temporary solution where we focused on testing tracking with gitlab-experiment. However, always enable tracking on specs has the side-effect that we can't test an implementation where we don't want to track.

It now calls the original implementation and respect when an experiment does not run for a certain actor or is not active.

Screenshots (strongly suggested)

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