Delete unused field `PipeEnv.pe_isHaskellishFile`

Differential Revision: https://phabricator.haskell.org/D774

compiler/main/DriverPipeline.hs

@@ -597,8 +597,7 @@ runPipeline stop_phase hsc_env0 (input_fn, mb_phase)
 
              isHaskellishFile = isHaskell start_phase
 
-             env = PipeEnv{ pe_isHaskellishFile = isHaskellishFile,
-                            stop_phase,
+             env = PipeEnv{ stop_phase,
                             src_filename = input_fn,
                             src_basename = basename,
                             src_suffix = suffix',

compiler/main/PipelineMonad.hs

@@ -50,7 +50,6 @@ instance Outputable PhasePlus where
 
 -- PipeEnv: invariant information passed down
 data PipeEnv = PipeEnv {
-       pe_isHaskellishFile :: Bool,
        stop_phase   :: Phase,       -- ^ Stop just before this phase
        src_filename :: String,      -- ^ basename of original input source
        src_basename :: String,      -- ^ basename of original input source