Remove extraneous length check from deneb forkchoice

[ralexstokes]

Remove duplicate check:

verify_blob_kzg_proof_batch checks that len(expected_kzg_commitments) == len(blobs) == len(proofs) so it is duplicate to have here.

I'd say we assume this functionality is part of the API of verify_blob_kzg_proof_batch and if so we can remove this check here and also under the validator.md guide.

What do we think @asn-d6 @jtraglia @hwwhww ?

[jtraglia]

I think that's a fair assumption. LGTM 👍

[hwwhww]

I git-blamed it. This helper had more assertions before the blob decoupling.

@ralexstokes
Unless we do want to extend validate_blob content later, it would be simpler if we just remove validate_blob and use verify_blob_kzg_proof_batch directly in is_data_available.

[dapplion]

PR title should be something meaningful