How do I remove an old device from iTunes account?

I am moving from a work-owned iPhone to a new personal iPhone. I backed up my old phone and restore it to factory settings and turned it in.

It is long gone now, but still shows up in my account list of iTunes devices and "Remove Device" is greyed out. How can I remove it so my new phone can be added?

Evidently, I did not sign out of iTunes on the old phone. What to do?

Thanks for any advice on this. Rik