Remove unused model property

app/models/service.py

@@ -498,10 +498,6 @@ class Service(JSONModel):
             key=lambda folder: folder['name'].lower(),
         )
 
-    @property
-    def can_upload_letters(self):
-        return self.has_permission('letter') and self.has_permission('upload_letters')
-
     @cached_property
     def all_template_folder_ids(self):
         return {folder['id'] for folder in self.all_template_folders}