Remove top nav helper and related code

What does this MR do and why?

Remove TopNav helper

The helper isn't needed any longer, so we can just remove it.

Remove unused nav_link helpers

They are not used any longer, let's get rid of them.

Remove GitLab TopNavMenuBuilder and related models

Interestingly Gitlab::Nav::TopNavMenuItem is still in use in the new sidebar (for the "Create New Issue / Repository, etc." dropdown). So we leave that one untouched.

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

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.