Remove obsolete TODO for p5.Image.prototype.mask

Author: yifanmai

src/image/p5.Image.js

@@ -629,10 +629,6 @@ p5.Image.prototype.copy = function(...args) {
  * http://blogs.adobe.com/webplatform/2013/01/28/blending-features-in-canvas/
  */
 // TODO: - Accept an array of alpha values.
-//       - Use other channels of an image. p5 uses the
-//       blue channel (which feels kind of arbitrary). Note: at the
-//       moment this method does not match native processing's original
-//       functionality exactly.
 p5.Image.prototype.mask = function(p5Image) {
   if (p5Image === undefined) {
     p5Image = this;