`TypedDictValidator` can drop support for `extra`

[samuelcolvin]

This dates back to when TypedDictValidator was used to build fields for models. extra is not supported by Python TypedDicts so we can remove the logic from here.

@sydney-runkle can we do that now, or do we need to wait for V3?

[samuelcolvin]

same might apply to dataclasses?

[sydney-runkle]

Yes, I think same applies. I think we can do this in V3 :).

[Viicos]

Depending on when we plan to release V3, I think it might be good to remove extra (at least for typed dicts) before, as we'll want to support PEP 728 which might get accepted soon (crossref the previous discussions we had on Slack with Sydney about this).

[sydney-runkle]

Wait, thinking about this more - don't we want to leave the infrastructure for extra support given PEP 728?