Remove gitaly_pack_objects_cache_enabled metric documentation

What does this MR do?

This removes a documentation reference to a Gitaly prometheus gauge that is being removed in gitaly!3672 (merged).

We used this gauge during the development of the feature, but I am not sure if it ever was valuable for administrators. So I consider this clean-up.

Related issues

gitlab-com/gl-infra/scalability#1126 (closed)

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Ensure that the product tier badge is added to doc's h1.
• Request a review based on the documentation page's metadata and associated Technical Writer.

To avoid having this MR be added to code verification QA issues, don't add these labels: feature, frontend, backend, ~"bug", or database

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.