unusable rows with blinding disabled

[ed255]

With #220 we have the option to mark some columns as unblinded.
Nevertheless, even if all columns are unblinded, we're still leaving unusable rows:

halo2/halo2_proofs/src/plonk/prover.rs

Line 300 in 0c3e3b5

let unusable_rows_start = params.n() as usize - (meta.blinding_factors() + 1);

Discussion from #243 (comment)