Remove mysterious hFlush stdout (#3825)

This has been there since before we open sourced DAML and afaict if it
has ever served any purpose, that purpose is long gone.

compiler/scenario-service/client/src/DA/Daml/LF/ScenarioServiceClient/LowLevel.hs

@@ -235,7 +235,6 @@ withScenarioService opts@Options{..} f = do
         -- if they are blocked in hGetNonBlocking so it is crucial that we close stdin in the
         -- callback or withAsync will block forever.
         flip finally (closeStdin stdinHdl) $ do
-            System.IO.hFlush System.IO.stdout
             port <- either fail pure =<< takeMVar portMVar
             liftIO $ optLogInfo $ "Scenario service backend running on port " <> show port
             -- Using 127.0.0.1 instead of localhost helps when our packaging logic falls over