Remove type alias WriterProperties.

I don't think this aids clarity, and anyway it's not exported.

src/Text/Pandoc/Scripting.hs

@@ -41,12 +41,11 @@ data ScriptingEngine = ScriptingEngine
     -- ^ Function to parse input into a 'Pandoc' document.
 
   , engineWriteCustom :: forall m. (PandocMonad m, MonadIO m)
-                      => FilePath -> m (WriterProperties m)
+                      => FilePath
+                      -> m (Writer m, ExtensionsConfig, m (Template Text))
     -- ^ Invoke the given script file to convert to any custom format.
   }
 
-type WriterProperties m = (Writer m, ExtensionsConfig, m (Template Text))
-
 noEngine :: ScriptingEngine
 noEngine = ScriptingEngine
   { engineName = "none"