Summary:
With `-XTypeInType`, the singletons machinery that `GHC.Generics` uses
no longer needs `KProxy` to hack around the use of `k` as both a type
and a kind. This is simply changing the code to match what's currently
in the upstream `singletons` repo.

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: thomie, goldfire

Differential Revision: https://phabricator.haskell.org/D2347