Domain Mapping login issue

I'm mapping domains in my multisite, i.e. Domain mapping, but without a plugin. The mapping seems to be working fine, but when I log in through the mapped domain, it still shows I'm not logged in through the original subsite URL. I guess it is supposed to sync, but seems to be not working. I just want to be able to log in through the mapped domain not both the mapped domain and original subsite URL at the same time. I tried putting the following in the wp-config.php file, but it didn't seem to work:  Any advices are appreciated.