Remove the item from the specified section.

        Intended for use with items of settings.ARMSTRONG_SECTION_ITEM_MODEL.
        Behavior on other items is undefined.

def remove_item(self, item, field_name=None):
        """
        Remove the item from the specified section.

        Intended for use with items of settings.ARMSTRONG_SECTION_ITEM_MODEL.
        Behavior on other items is undefined.
        """
        field_name = self._choose_field_name(field_name)
        related_manager = getattr(item, field_name)
        related_manager.remove(self)