Remove staticmethod.__new__ and classmethod.__new__

[JelleZijlstra]

It's not clear from the stub why these exist; they just have *args/**kwargs and a generic signature.

Does anyone know of a good reason for these to be in the stub?

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[AlexWaygood]

Seems like we can do without them!

[JelleZijlstra]

It was added in #1089.

At runtime it really does accept any args, but creates a useless object:

In [12]: cm=classmethod.__new__(classmethod, len, x=3); print(cm.__get__(int))
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-12-e2ad981a1fb1> in <module>
----> 1 cm=classmethod.__new__(classmethod, len, x=3); print(cm.__get__(int))

RuntimeError: uninitialized classmethod object

I don't know what that's good for and it doesn't seem important to reflect it in the stub.