Portfolio

I believe you do it like this. In the work plan of the Portfolio select the required project. On the Misc menu select Detach. This will turn the selected project into a standalone project.

That works - I found the detach option under the "Other Actions" menu.

Great news.

I believe this information has changed. I was testing this out today and could not find a Misc menu or Detach option anywhere. I did simply select a project I wanted to remove from the portfolio and chose the Delete option which asked if I was sure I wanted to delete the selected shortcut. I answered yes and the link to the project was removed from my Portfolio project. I verified the project whose shortcut I deleted still existed (it did.)