Revert "Define functools.partial as overloaded function instead of its own class"

[JelleZijlstra]

Reverts #2878

Caused problems in many real-world codebases. Proper support for partial will have to come from a plugin instead.

I've seen the following problems reported:

• Keyword arguments to partial() functions no longer work
• isinstance() checks on functools.partial no longer work
• Access to .func and other attributes no longer works