Bug 1357631 - Drop comments about updating the type of `target` arguments/members in KeyframeEffect(ReadOnly); r=bz DONTBUILD

In bug 1241783 and Github issue #186[1] it was decided to update the WebIDL in
the Web Animations spec so that instead of using a mixin as the type, we instead
use a suitable union type.

This patch removes the comments that indicated that the Gecko WebIDL definition
need to be updated to match the spec, since the spec has now been updated to
match the Gecko WebIDL definition instead.[2]

[1] https://github.com/w3c/web-animations/issues/186
[2] a07f609270
    d0445d2a46

dom/webidl/KeyframeEffect.webidl

@@ -21,17 +21,12 @@ dictionary KeyframeEffectOptions : AnimationEffectTimingProperties {
   DOMString                   spacing = "distribute";
 };
 
-// Bug 1241783: For the constructor we use (Element or CSSPseudoElement)? for
-// the first argument since we cannot convert a mixin into a union type
-// automatically.
 [Func="nsDocument::IsWebAnimationsEnabled",
  Constructor ((Element or CSSPseudoElement)? target,
               object? keyframes,
               optional (unrestricted double or KeyframeEffectOptions) options),
  Constructor (KeyframeEffectReadOnly source)]
 interface KeyframeEffectReadOnly : AnimationEffectReadOnly {
-  // Bug 1241783: As with the constructor, we use (Element or CSSPseudoElement)?
-  // for the type of |target| instead of Animatable?
   readonly attribute (Element or CSSPseudoElement)?  target;
   readonly attribute IterationCompositeOperation iterationComposite;
   readonly attribute CompositeOperation          composite;