Remove this when we support negatives

[recmo]

On 2020-05-06 @pvienhage wrote in af4b6e2 “Basic autogeneration (#687)”:

Remove this when we support negatives

    // We map each trace to the row it contains
    let mut rows = trace_keys
        .iter()
        .map(|expression| {
            match expression {
                // TODO - Remove this when we support negatives
                #[allow(clippy::cast_sign_loss)]
                RationalExpression::Trace(_, j) => *j as usize,
                _ => panic!("Non trace in trace array"),
            }
        })

From crypto/stark/src/solidity_verifier.rs:298