docs(contributing): remove the dead GitPOAP section

[Gabriel-Dalton]

Removes the "Claiming GitPOAP" section and its table of contents entry from CONTRIBUTING.md. gitpoap.io no longer resolves and there is no replacement place to claim, so the section is removed rather than redirected, as suggested in the issue.

Closes #1506

[greptile-apps]

Confidence Score: 5/5

[Low risk] Removes outdated contributor documentation section.

This documentation-only change appears safe to merge.

What we checked:

• Links to the removed section: No remaining repository link points to the removed heading. The contents list also removes that entry and retains working links to neighboring sections.

Summary

Removes the obsolete GitPOAP claim instructions, image, and matching table-of-contents entry from CONTRIBUTING.md.

• The remaining contents links still match their headings.
• No actionable issues or rule violations were found.

_{Reviews (1) · Last reviewed commit: "docs(contributing): remove the dead GitP..." · Reviewed by Greptile}