It's hard to say exactly what reference to Tableau Server has made it into your workboook and which one is causing the login prompt behavior. Someone will need to explore the XML inside the workbook (you can do this too, with a copy, of course!) You'll probably have to post it or open up a case with Tableau Support if you really want to figure it out.
Thanks. I'll open up a case. I was assuming it was some setting I wasn't aware of.