test: fix signature SSZ test vectors and remove irrelevant SSZ test vectors

[unnawut]

🗒️ Description

Some final fixes to get all test vectors working with client implementations.

• Fix existing signature test vectors that was using its own mock signature scheme so it couldn't pass on client implementations that's unaware of this mock setup. Now it uses signatures from consensus_testing.keys which generate actual keys and signatures according to --scheme flag
• Remove SSZ test vectors for SecretKey, HashTreeOpening, HashTreeLayer, HashSubTree because they're currently not used as SSZ containers so clients may not have them implemented.

🔗 Related Issues or PRs

• A follow up of fix: serialize signatures to fixed bytes #435

✅ 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.