Remove `verify_cell_kzg_proof` (non-batch)

[jtraglia]

The specs and clients are only expected to use verify_cell_kzg_proof_batch, so we could remove verify_cell_kzg_proof. This would simplify things quite a bit. In ckzg, we could delete ~200 lines of C code and a bunch of code in the bindings. It's still possible to verify a single cell with the batch function and the performance is essentially the same.