Remove UninferredClosure type

[vsmenon]

These are cases that should be handled by contextual inference, but are not. We appear to still generate these though. It's a warning.

@leafpetersen : do you know what we're not handling yet?

[leafpetersen]

My guess is because I have only implemented downwards inference to functions of the form (x) => e, and not of the form (x) => {...}.

[jmesserly]

https://codereview.chromium.org/1627503002/