Remove thread counts for Gitaly Ruby workers in ref. arch. docs

What does this MR do?

The number of threads for Gitaly Ruby workers isn't valid and should be removed. Per Grant's comment: !18604 (comment 231732142)

Related issues

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

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3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set and that you merge the equivalent EE MR before the CE MR if both exist.
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