Remove Frequently used Git commands page from global navigation

What does this MR do and why?

For: gitlab#419396 (closed), and along with gitlab!140169 (merged), remove a now redirected page from global navigation.

Because this was the parent item for sub pages in the navigation, flatten the navigation a bit.

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/425355

Screenshots, screen recordings, or links to review app

• Before: https://docs.gitlab.com/ee/topics/git/
• After: https://eread-remove-redirected-frequently-used-git-commands-page-from.docs.gitlab-review.app/ee/topics/git/

Merge request acceptance checklist

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