Remove Proxy from promoted classes

[goldfirere]

In GHC8, there should be no reason to use Proxy in a promoted class. But efforts at removing this proxy have been stymied. I've not investigated fully, but here is what I know:

• The problem seems unrelated to TH. When I copied spliced code back into the buffer, the error did not change.
• There's a chance that the error is due to CUSKiness: there's a definition which looks CUSKy but I want non-CUSK inference.
• Further probing led to Not in scope while type-checking ghc#69. So I gave up. But I doubt that's the real reason for the error.

In any case, I'm pretty sure this is blocked upstream.