New unarise (714bebff) eliminates void binders in patterns already, so no
need to eliminate them here. I leave assertions to make sure this is the
case.

Assertion failure -> bug in unarise

Reviewers: bgamari, simonpj, austin, simonmar, hvr

Reviewed By: simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2416