If you want Visual Studio 2017 to ALWAYS prune on fetch (I do, I add and remove a LOT of branches for UI work), once you install 3rd party Git (can do from Settings within Team Explorer, Git section) there will be a new option in Global Settings (also in Git section) to ‘Prune remote branches during fetch’ which you can set to True. I did not have this option before updating my VS to 15.5 (I was on 15.0).
