Remove New Nav Toggle code

What does this MR do and why?

Remove New Nav Toggle code

This probably should have been removed in !134531 (merged) as well, but let's just remove it now.

Screenshots or screen recordings

N/A

How to set up and validate locally

There is no way to to have the old nav enabled, so we can remove this bit as well.

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.