dkr-remove problem

[jacksoom]

Hi teams.
After I use dkr-removal, I use the removed keypair to sign and it is normal.

Test case code

fn test_remove_sign_rotate_sign() {
        let mut keys = simulate_keygen(2, 5);
        let offline_sign = simulate_offline_stage(keys.clone(), &[1, 2, 3]);
        simulate_signing(offline_sign, b"ZenGo");
        simulate_dkr_removal::<{ M_SECURITY }>(&mut keys, [1].to_vec());
        let offline_sign = simulate_offline_stage(keys.clone(), &[1, 3, 4]); // index 1 has been removed
        simulate_signing(offline_sign, b"ZenGo");
        simulate_dkr_removal::<{ M_SECURITY }>(&mut keys, [1, 2].to_vec());
        let offline_sign = simulate_offline_stage(keys, &[3, 4, 5]);
        simulate_signing(offline_sign, b"ZenGo");
    }