implement delete attribute by reorganize hidden class

Well, it's partially correct -- whether the instance attribute gets checked before the class attribute depends on whether the class attribute is a *data descriptor* (defines both \_\_get__ and \_\_set__).  If it's a *non-data descriptor* then it gets checked after the instance attributes.

This trickiness is why descriptors aren't implemented yet, and why I don't think it would be bad to just ignore them here for now :)