Remove duplicate install GitLab navigation item

What does this MR do and why?

Removes duplicate "install GitLab" navigation item that causes the menu to not render correctly:

The MVC was to remove the sub-item and bring its own sub-items up a level:

• Before: https://docs.gitlab.com/ee/install/
• After: http://eread-remove-duplicate-install-gitlab-nav-item.35.193.151.162.nip.io/ee/install/

Merge request acceptance checklist

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