Remove CSS.unregisterProperty as the WG resolved to remove it for now

third_party/WebKit/Source/core/css/PropertyRegistration.h

 // Copyright 2016 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.
 
 #ifndef PropertyRegistration_h
 #define PropertyRegistration_h
 
 #include "wtf/Allocator.h"
 #include "wtf/text/WTFString.h"
 
 namespace blink {
 
 class ExceptionState;
 class ExecutionContext;
 class PropertyDescriptor;
 
 class PropertyRegistration {
   STATIC_ONLY(PropertyRegistration);
 
  public:
   static void registerProperty(ExecutionContext*,
                                const PropertyDescriptor&,
                                ExceptionState&);
-  static void unregisterProperty(ExecutionContext*,
-                                 const String&,
-                                 ExceptionState&);
 };
 
 }  // namespace blink
 
 #endif  // PropertyRegistration_h