Bug 1402170 - Drop invalid test for offsets; r=hiro

There is a test that assumes that an offset specified on a property-indexed
keyframe is applied to all generated keyframes but that behavior is not (yet)
specified.

This behavior will be specified in [1] but until that happens it seems invalid
to test for it. Furthermore, when that is specified we will need much more
thorough tests than this one.

[1] https://github.com/w3c/web-animations/issues/148

MozReview-Commit-ID: HUUw88dg2P7

--HG--
extra : rebase_source : 5e38d8f0fb01b3ecf7339ca1be0e31c775bf4b21

testing/web-platform/meta/web-animations/interfaces/KeyframeEffect/processing-a-keyframes-argument.html.ini

@@ -1,5 +0,0 @@
-[processing-a-keyframes-argument.html]
-  type: testharness
-  [Equivalent property indexed and sequenced keyframes: same offset applied to all keyframes]
-    expected: FAIL
-

testing/web-platform/tests/web-animations/interfaces/KeyframeEffect/processing-a-keyframes-argument.html

@@ -124,17 +124,6 @@ const gEquivalentSyntaxTests = [
       {left: '200px', opacity: '1'},
     ],
   },
-  {
-    description: 'same offset applied to all keyframes',
-    indexedKeyframes: {
-      left: ['0px', '100px'],
-      offset: 0.5,
-    },
-    sequencedKeyframes: [
-      {left: '0px', offset: 0.5},
-      {left: '100px', offset: 0.5},
-    ],
-  },
   {
     description: 'same easing applied to all keyframes',
     indexedKeyframes: {