Remove locked field from Main Form

Hello,

I need to customize Case entity in online CRM 2016, I already added my custom field. 

when come to forms, I need to create new one, the system give me a form with fields already I customized it.

but some field is locked and I can't change it to "unlock field" and I cant remove it from the form.

I add new section and make it invisible by default and added all that field in it, when I try to work on that form, the form can't be submitted and it ask for those hidden field.

please any help?

thanks