Remove Unstage files nav item from Git section of global navigation

What does this MR do and why?

A companion MR to gitlab!149364 (merged), where we are combining two topics.

Relates to: gitlab#451591.

• Before: https://docs.gitlab.com/ee/topics/git/undo.html
• After: https://eread-remove-unstage-nav-item-from-git-section.docs.gitlab-review.app/ee/topics/git/undo.html

Merge request 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 merge request.