[Extensions] Remove JSONFeatureProvider, SimpleFeature::Parse

extensions/common/features/api_feature.cc

-// 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 "extensions/common/features/api_feature.h"
-
-namespace extensions {
-
-APIFeature::APIFeature() {}
-
-APIFeature::~APIFeature() {
-}
-
-bool APIFeature::Validate(std::string* error) {
-  if (!SimpleFeature::Validate(error))
-    return false;
-
-  if (contexts().empty()) {
-    *error = name() + ": API features must specify at least one context.";
-    return false;
-  }
-
-  return true;
-}
-
-}  // namespace extensions