Cleanup outliner instancing collection code.

Remove needless call to `id_lib_extern`, this is already part of
`id_ud_plus` code.

source/blender/editors/space_outliner/outliner_collections.cc

@@ -799,7 +799,6 @@ static int collection_instance_exec(bContext *C, wmOperator * /*op*/)
         C, OB_EMPTY, collection->id.name + 2, scene->cursor.location, nullptr, false, 0);
     ob->instance_collection = collection;
     ob->transflag |= OB_DUPLICOLLECTION;
-    id_lib_extern(&collection->id);
     id_us_plus(&collection->id);
   }