gitaly: Remove gitaly-ruby related code

What does this merge request do and why?

Gitaly dropped all use of Ruby, so now we can remove all code that sets up Ruby gems.

Issue: gitaly#4742 (closed)

How to set up and validate locally

1. Bring <gdk>/gitaly to a clean state: git -C gitaly clean -Xfd
2. Set up Gitaly again: make gitaly-setup
3. Start Gitaly and see if it works: gdk restart gitaly && gdk tail gitaly

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.