Remove more now redirected Git-related pages from global navigation

What does this MR do and why?

For gitlab#468098, and as a companion to: gitlab!157251 (merged), let's continue to remove navigation items from the global navigation for topics that are now redirected. This helps simplify the information architecture of this section of pages.

Merge request acceptance checklist

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