Delete unused AnimatableValue code

third_party/WebKit/Source/core/animation/animatable/AnimatableValueKeyframe.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/animation/animatable/AnimatableValueKeyframe.h"
 
 #include "core/animation/LegacyStyleInterpolation.h"
 
 namespace blink {
 
@@ skipping 35 matching lines @@
 
 PassRefPtr<Interpolation>
 AnimatableValueKeyframe::PropertySpecificKeyframe::createInterpolation(
     const PropertyHandle& property,
     const Keyframe::PropertySpecificKeyframe& end) const {
   return LegacyStyleInterpolation::create(
       value(), toAnimatableValuePropertySpecificKeyframe(end).value(),
       property.cssProperty());
 }
 
-PassRefPtr<Keyframe::PropertySpecificKeyframe>
-AnimatableValueKeyframe::PropertySpecificKeyframe::neutralKeyframe(
-    double offset,
-    PassRefPtr<TimingFunction> easing) const {
-  return create(offset, std::move(easing), AnimatableValue::neutralValue(),
-                EffectModel::CompositeAdd);
-}
-
 }  // namespace blink