implementation of Default for FlexParams is invalid

[andrewhickman]

#1691 added a check to prevent using a flex factor less than or equal to 0.0, but FlexParams has #[derive(Default)] which allows bypassing the check with FlexParams::default().

Should we remove the implemenation or does it make sense to use a value like 1.0?

[cmyr]

Great question. Before that patch, passing FlexParams::default() would result in a non-flex child. I think what makes the most sense is to remove the default impl.