Revert "Build the substitution correctly in piResultTy"
This reverts commit dbf72dbc. This commit introduced performance regressions: https://ghc.haskell.org/trac/ghc/ticket/11371#comment:27, I will push it again after I fix it. Test Plan: revert Reviewers: simonpj, bgamari, simonmar, austin, goldfire, thomie Differential Revision: https://phabricator.haskell.org/D1907
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