Remove @keyframe parsing.

We still have the code internally, but the parser won't create
StyleRuleKeyframes anymore.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/871773003

engine/src/flutter/tests/animation/basic-declarative-expected.txt

@@ -1,5 +0,0 @@
-Running 1 tests
-ok 1 top should amimate from 0 to 100px
-1 tests
-1 pass
-0 fail

engine/src/flutter/tests/animation/basic-declarative.sky

@@ -1,31 +0,0 @@
-<sky>
-<import src="../resources/chai.sky" />
-<import src="../resources/mocha.sky" />
-<div id="example"></div>
-<style>
-#example {
-    width: 100px;
-    height: 100px;
-    background: red;
-    position: relative;
-    animation-name: anim;
-    animation-timing-function: linear;
-    animation-duration: 5s;
-}
-
-@keyframes anim {
-    0%   { top: 0px; }
-    100% { top: 100px; }
-}
-</style>
-<script>
-var example = document.getElementById('example');
-
-describe('top', function() {
-  it('should amimate from 0 to 100px', function() {
-    internals.pauseAnimations(3);
-    assert.equal('60px', getComputedStyle(example).top);
-  });
-});
-</script>
-</sky>