CmmExpr: remove unused `vgcFlag` function
CmmExpr: remove unused `vgcFlag` function

Test Plan: validate

Reviewers: austin, bgamari, simonmar

Reviewed By: bgamari, simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2351

shlevy/ghc,sdiehl/ghc,snoyberg/ghc,sdiehl/ghc,sgillespie/ghc,sgillespie/ghc,mettekou/ghc,shlevy/ghc,ezyang/ghc,olsner/ghc,sgillespie/ghc,snoyberg/ghc,olsner/ghc,vTurbine/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,mettekou/ghc,snoyberg/ghc,snoyberg/ghc,vTurbine/ghc,ezyang/ghc,olsner/ghc,ezyang/ghc,ezyang/ghc,sgillespie/ghc,sgillespie/ghc,snoyberg/ghc,vTurbine/ghc,sgillespie/ghc,vTurbine/ghc,vTurbine/ghc,mettekou/ghc,mettekou/ghc,sdiehl/ghc,sdiehl/ghc,olsner/ghc,vTurbine/ghc,snoyberg/ghc,ezyang/ghc,vTurbine/ghc,olsner/ghc,olsner/ghc,mettekou/ghc,shlevy/ghc,snoyberg/ghc,sgillespie/ghc,shlevy/ghc,olsner/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,mettekou/ghc,sdiehl/ghc,mettekou/ghc