#15548 - False Positive Related Object in Custom Fields

[Julio-Oliveira-Encora]

Fixes: #15548

I created the class CustomObjectDeleteView, which was inherited from ObjectDeleteView, and overwritten the "get" method and its dependent_objects variable. This way, the modal from delete_form.html will not display messages about dependent objects.

Also, I changed the inherited class for CustomFieldDeleteView to use generic.CustomObjectDeleteView.