remove unused isSymLink

That made Utility.FileMode depend on unix, but Utility.Tmp now depends
on it, and is used by Setup, which does not. So it was easiest to remove
this, especially since it's not used.

Utility/FileMode.hs

@@ -1,6 +1,6 @@
 {- File mode utilities.
  -
- - Copyright 2010-2017 Joey Hess <id@joeyh.name>
+ - Copyright 2010-2020 Joey Hess <id@joeyh.name>
  -
  - License: BSD-2-clause
  -}
@@ -16,11 +16,7 @@ import System.IO
 import Control.Monad
 import System.PosixCompat.Types
 import System.PosixCompat.Files
-#ifndef mingw32_HOST_OS
-import System.Posix.Files (symbolicLinkMode)
-import Control.Monad.IO.Class (liftIO)
-#endif
-import Control.Monad.IO.Class (MonadIO)
+import Control.Monad.IO.Class
 import Foreign (complement)
 import Control.Monad.Catch
 
@@ -97,14 +93,6 @@ groupWriteRead f = modifyFileMode f $ addModes groupSharedModes
 checkMode :: FileMode -> FileMode -> Bool
 checkMode checkfor mode = checkfor `intersectFileModes` mode == checkfor
 
-{- Checks if a file mode indicates it's a symlink. -}
-isSymLink :: FileMode -> Bool
-#ifdef mingw32_HOST_OS
-isSymLink _ = False
-#else
-isSymLink = checkMode symbolicLinkMode
-#endif
-
 {- Checks if a file has any executable bits set. -}
 isExecutable :: FileMode -> Bool
 isExecutable mode = combineModes executeModes `intersectFileModes` mode /= 0