Remove unnecessarily returned res_ty from rejigConRes

@goldfire noticed that we don't need to thread through `res_ty`
through to the return type of `rejigConRes`, as it never changes.

Reviewers: goldfire, austin, bgamari

Reviewed By: goldfire

Subscribers: rwbarton, thomie, goldfire

Differential Revision: https://phabricator.haskell.org/D3725