Match signature of Model.delete()

`delete(self)` will work, but at least PyCharm will throw a warning `Signature of method MyClass.delete() does not match signature of base class method in class 'Model'`. Changing to `def delete(self, *args, **kwargs):` fixes this.
Recommended to use using=None
Merge pull request #19 from SaeX/patch-1

Match signature of Model.delete()