Removing the audit flag of a custom field

I wanted to remove the audit flag of a custom field in the opportunity module, but although I can remove it, after a 'save' it always returns.

Is this caused by this line in the custom\Extension\modules\Opportunities\Ext\Vardefs  folder ? 

"$dictionary['Opportunity']['fields']['opportunity_class_c']['audited'] = true;"

Thanks for your feedback,

Hugo