Revert "Treat varargs as legal context in default lambda argument"

[gvanrossum]

Reverts #2770

Here's a snippet that now crashes:

import warnings
warnings.warn = lambda *args, **kwargs: None

(This is literally from our codebase, except there we have a # type: ignore too.)

[gvanrossum]

Sorry @elazarg and @JukkaL -- this was getting in the way of our internal CI. I'm sure it's easy enough to figure out an improved version given the example.