Remove duplicate navigation item for installing GitLab Runner

What does this MR do and why?

Because duplicate entries don't render correctly on the global nav, I've removed a duplicate entry for runner/install/.

Because the structure is now much flatter, I've tried to provide a bit more context for navigation items so they're less ambiguous and are hopefully improved!

Screenshots, screen recordings, or links to review app

• Before: https://docs.gitlab.com/runner/install/

  

• After: http://eread-remove-duplicate-install-gitlab-runner-nav-entry.35.193.151.162.nip.io/runner/install/

  

Merge request acceptance checklist

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