doc/gitaly: Remove references to removed metrics

What does this MR do and why?

In Add Metrics Dashboard menu (!121742 - merged), we fixed a bug where a menu items that had been missed when migrating to the new navigation was re-added.

This MR backports that fix to v15.11

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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