Remove the Gradient effect type for now

It's troublesome to implement and will probably look kind of weird anyway.

ShareX.HelpersLib/Enums.cs

@@ -218,7 +218,6 @@ public enum CutOutEffectType // TODO: localize
         None,
         ZigZag,
         TornEdge,
-        Wave,
-        Gradient
+        Wave
     }
 }

ShareX.HelpersLib/Helpers/ImageHelpers.cs

@@ -238,9 +238,6 @@ private static Bitmap ApplyCutOutEffect(Bitmap bmp, AnchorStyles effectEdge, Cut
 
                 case CutOutEffectType.Wave:
                     return WavyEdges(bmp, effectSize, effectSize * 5, effectEdge);
-
-                case CutOutEffectType.Gradient:
-                    return bmp;
             }
 
             throw new NotImplementedException(); // should not be reachable