[GroupEffect] Remove SingleKeyframeEffectAnimation

SingleKeyframeEffectAnimation was introduced to support multiple effects
per animation for AnimationWorklet. i.e. each animation has a list of
single KeyframeEffects.

However, GroupEffect is now re-designed for WebAnimation. Instead of a
list of KeyframeEffects, an animation will have one AnimationEffect
which could be either a single KeyframeEffect of a GroupEffect which
combines a list of AnimationEffect in different manners. See [1] for
details. Therefore the existing model is obsolete and should be removed.

This patch is just a refactoring and it should not affect any prod
paths.

[1] https://github.com/yi-gu/group_effect

Bug: 767043, 956696
Change-Id: I2b5bc60dea9763ac64e5484e6ba793d8b6404089
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993429
Commit-Queue: Yi Gu <yigu@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730563}

cc/animation/BUILD.gn

@@ -40,8 +40,6 @@ cc_component("animation") {
     "scroll_offset_animations_impl.h",
     "scroll_timeline.cc",
     "scroll_timeline.h",
-    "single_keyframe_effect_animation.cc",
-    "single_keyframe_effect_animation.h",
     "timing_function.cc",
     "timing_function.h",
     "transform_operation.cc",

cc/animation/README.md

@@ -32,15 +32,7 @@ property of the same target across multiple Animations.
 Each Animation has a copy on the impl thread, and will take care of
 synchronizing to/from the impl thread when requested.
 
-[SingleKeyframeEffectAnimation](https://cs.chromium.org/chromium/src/cc/animation/single_keyframe_effect_animation.h)
-It is a sub-class of Animation that serves as a bridge between the cc animation
-clients and cc because currently only a single keyframe effect per animation is
-supported.
-
-There is a 1:1 relationship between SingleKeyframeEffectAnimation and
-the KeyframeEffect.
-
-> NOTE: SingleKeyframeEffectAnimation is being deprecated.
+There is a 1:1 relationship between Animation and KeyframeEffect.
 
 [Keyframe model](https://codesearch.chromium.org/chromium/src/cc/animation/keyframe_model.h)
 KeyframeModels contain the state necessary to 'play' (interpolate values from) an