Remove unused methods from sidebar panel classes

What does this MR do and why?

This MR is part of the Remove the old navigation code (&11875) epic.

This MR removes a bunch of now unused methods in the lib/sidebars classes.

Screenshots or screen recordings

No visual change expected.

How to set up and validate locally

• Make sure CI is passing.
• Go through the removed methods and double-check what they were used for in the legacy nav and that they weren't serialized into the data blob for the new nav (which is rendered with Vue.js)
• Reach out to me or @leipert (or anyone in the Foundations team) if you are not familiar with this area of the code.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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