remove dead function 'tcInstBinders'
remove dead function 'tcInstBinders'

ezyang/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc