restart: Don't pass error as unused user data

We passed the pointer to a GError * as user data on an async I/O call.
The callback function didn't make use of it, so it was never written to,
thus remained NULL, thus was dead code. Remove it.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2446>

src/core/restart.c

@@ -174,13 +174,7 @@ meta_restart (const char *message)
 
   g_data_input_stream_read_line_async (data_stream, G_PRIORITY_DEFAULT,
                                        NULL, restart_helper_read_line_callback,
-                                       &error);
+                                       NULL);
-  if (error != NULL)
-    {
-      meta_warning ("Failed to read from restart helper: %s", error->message);
-      g_object_unref (data_stream);
-      goto error;
-    }
 
   return;