Really, there's no need to do anything. Cities that aren't played on for a while get hidden from view anyway.
Further, you can play on as many worlds as you want, all on one account. So you don't have to worry about anything bad happening.
On top of that, diamonds are stored on your base account, not per world. So you have your diamonds wherever you are.
Hope that helps. If you still want to delete your city, I'd send an in-game support ticket asking them. Right-click in-game, "Contact Support", "Contact Us"