testing framework: rm unused SignedAttestationSpec

[tcoratger]

🗒️ Description

🔗 Related Issues or PRs

Followup after #321 cc @unnawut

✅ Checklist

• Ran tox checks to avoid unnecessary CI fails:

  uvx tox

• Considered adding appropriate tests for the changes.
• Considered updating the online docs in the ./docs/ directory.

[unnawut]

I spotted this too while working on #321 but didn’t remove because I was hoping I could get to the point where we have test vectors for ssz and gossipsub as well, where then we need to create individual signed attestations again

but since you also have an itch on this, I agree we can remove and always add back when we actually need it