Remove legacy sidebar tracking code

What does this MR do and why?

Remove legacy sidebar tracking code

The code wasn't used any longer, in fact with the new nav query selector: .nav-sidebar li[data-track-label] > a seems to be always empty.

Screenshots or screen recordings

N/A

How to set up and validate locally

Evaluate document.querySelector('.nav-sidebar li[data-track-label] > a') on any page, it should return null. Therefore this code was never executed with the new nav.

MR acceptance checklist

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