It's a woodruff key. It should come out, they are sometime fairly tight. Soak it in the best penetrating fluid you can get your hands on for a bit. If you can get hold of a shaft or punch non-metallic or soft faced to strike it with that won't mar the surface whilst holding the charger casing firmly to avoid damage that should do it. Do it on an angle so it's trying to knock the key away from the shaft.