Javascript in website source code?

That originates from one of the settings in your Network: Settings > Domain Mapping (/wp-admin/network/settings.php?page=dm_admin_page)

Starting with this one.

1. Redirect administration pages to site’s original domain (remote login disabled if this redirect is disabled)

I could use more (updated) documentation on this, as well, to understand what this is doing, beyond what is found on this plugin’s “Installation” page.

https://ww.wp.xz.cn/plugins/wordpress-mu-domain-mapping/installation/

And this background page linked from there:

http://ottopress.com/2010/wordpress-3-0-multisite-domain-mapping-tutorial/

“Remote Login – This will make your login pages for all sites redirect to your main site to do the actual login. The benefit of this is that when you log in to one, you log into all of them. The downside is that the URL changes to another domain in order to log in.”