[arabic] Disable 'cswh' by default

I believe Windows 8 disables it, and spec update dated
Jan 2014 also clearly says it's disabled by default:

  http://www.microsoft.com/typography/OpenTypeDev/arabic/intro.htm#features

src/hb-ot-shape-complex-arabic.cc

@@ -199,7 +199,6 @@ collect_features_arabic (hb_ot_shape_planner_t *plan)
   map->add_global_bool_feature (HB_TAG('c','a','l','t'));
   map->add_gsub_pause (NULL);
 
-  map->add_global_bool_feature (HB_TAG('c','s','w','h'));
   map->add_global_bool_feature (HB_TAG('m','s','e','t'));
 }