PYTHON-4755 - Stop supporting and testing against Eventlet

[NoahStapp]

Summary

Eventlet is deprecated, and we should not continue supporting and testing against it.

Changes in this PR

Remove Eventlet testing variants and code specific to Eventlet.

Test Plan

Run a full test matrix to verify no regressions.

Screenshots (optional)

N/A

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is the intention of the code captured in relevant tests?
• If there are new TODOs, has a related JIRA ticket been created? (PYTHON-5573)

Checklist for Reviewer

• [] Does the title of the PR reference a JIRA Ticket?
• [] Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• [] Have you checked for spelling & grammar errors?
• [] Is all relevant documentation (README or docstring) updated?

Focus Areas for Reviewer

• Are code changes limited to Eventlet-specific logic and functionality?

[blink1073]

🫡 . LGTM!