Remove unused surface property

This helps with the Qt6 build, as  KWaylandServer::SurfaceInterface is a
QObject subclass used in a property here, while we are only able to
forward declare it here as it comes from a library we don't link against.
And that's no longer supported in Qt6.

src/libkwineffects/kwineffects.h

@@ -2233,12 +2233,6 @@ class KWINEFFECTS_EXPORT EffectWindow : public QObject
      */
     Q_PROPERTY(bool skipsCloseAnimation READ skipsCloseAnimation)
 
-    /**
-     * Interface to the corresponding wayland surface.
-     * relevant only in Wayland, on X11 it will be nullptr
-     */
-    Q_PROPERTY(KWaylandServer::SurfaceInterface *surface READ surface)
-
     /**
      * Whether the window is fullscreen.
      * @since 5.6