Remove code related to old-style classes

[akamat10]

To simplify the astroid, shall we get rid of code in the repo related to old-style classes? This is tied to python 2.x and is no longer relevant. This will simplify the code base, improve maintainability and perhaps even have some performance improvements. ClassDef class is most affected by it. Happy to work on this.

[DanielNoord]

I'd be okay with it, seems like a nice improvement.

@Pierre-Sassoulas Do you agree?

[Pierre-Sassoulas]

Yeah, sounds good !

[DanielNoord]

I'd be okay with it, seems like a nice improvement.

@Pierre-Sassoulas Do you agree?

[Pierre-Sassoulas]

Yeah, sounds good !