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

third_party/WebKit/Source/core/css/PropertyRegistry.cpp

 // 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.
 
 #include "core/css/PropertyRegistry.h"
 
 namespace blink {
 
 void PropertyRegistry::registerProperty(
     const AtomicString& name,
     const CSSSyntaxDescriptor& syntax,
     bool inherits,
     const CSSValue* initial,
     PassRefPtr<CSSVariableData> initialVariableData) {
   DCHECK(!registration(name));
   m_registrations.set(
       name, new Registration(syntax, inherits, initial, initialVariableData));
 }
 
-void PropertyRegistry::unregisterProperty(const AtomicString& name) {
-  DCHECK(registration(name));
-  m_registrations.remove(name);
-}
-
 const PropertyRegistry::Registration* PropertyRegistry::registration(
     const AtomicString& name) const {
   return m_registrations.get(name);
 }
 
 }  // namespace blink