Remove thin and eventmachine from Ruby troubleshooting

What does this Merge Request do and why?

Removes thin and eventmachine gem sections from Ruby troubleshooting.

Merge Request checklist

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

Closes #1511 (closed)