How to remove class set by Form Field Definition Rule?

[Confi User]

I assign a field the class .cf_validation_error, when a validation rule is met, but how do I remove this when the rule is not met? I added this through the "Apply CSS on a field" in the Form Field Definition Rule. The instrutions say to do: 

class:cf_validation_error, but it does not say to remove classes.

Thanks :)