unusable rows with blinding disabled

[ed255]

With https://github.com/privacy-scaling-explorations/halo2/pull/220 we have the option to mark some columns as unblinded. Nevertheless, even if all columns are unblinded, we're still leaving unusable rows: https://github.com/privacy-scaling-explorations/halo2/blob/0c3e3b569519b86653a64f412bbce17e4e8acac4/halo2_proofs/src/plonk/prover.rs#L300

Discussion from https://github.com/privacy-scaling-explorations/halo2/pull/243#discussion_r1452121055