Remove all js-onboarding ids

What does this MR do and why?

This MR is part of &11875.

Remove all js-onboarding ids

These were used for an onboarding tour that was never enabled on production.

See !35442 (merged)

Screenshots or screen recordings

No visual change.

How to set up and validate locally

Browse a few pages in GDK and make sure the sidebar still renders. That's it 🙂

MR acceptance checklist

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