If your itinerary is all within Europe, then the legs are generally seen as one way tickets. And yes, you can get SK to remove the first leg, I have done this with Plus fares and even gotten a refund for the first leg. However, don't be a no show, you need to manage it in advance. Once your ticket include a long haul (as here) this does not work, and you have to fly the first leg to not get the ticket cancelled.