Remove some dead code

compiler/utils/Exception.hs

@@ -74,19 +74,11 @@ class Monad m => ExceptionMonad m where
       _ <- sequel
       return r
 
-#if __GLASGOW_HASKELL__ < 613
-instance ExceptionMonad IO where
-  gcatch    = catch
-  gmask f   = block $ f unblock
-  gblock    = block
-  gunblock  = unblock
-#else
 instance ExceptionMonad IO where
   gcatch    = catch
   gmask f   = mask (\x -> f x)
   gblock    = block
   gunblock  = unblock
-#endif
 
 gtry :: (ExceptionMonad m, Exception e) => m a -> m (Either e a)
 gtry act = gcatch (act >>= \a -> return (Right a))