[turbofan] Remove the JSContextRelaxation reducer.

src/compiler/js-context-relaxation.h

-// Copyright 2014 the V8 project 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 V8_COMPILER_JS_CONTEXT_RELAXATION_H_
-#define V8_COMPILER_JS_CONTEXT_RELAXATION_H_
-
-#include "src/compiler/graph-reducer.h"
-
-namespace v8 {
-namespace internal {
-namespace compiler {
-
-// Ensures that operations that only need to access the native context use the
-// outer-most context rather than the specific context given by the AST graph
-// builder. This makes it possible to use these operations with context
-// specialization (e.g. for generating stubs) without forcing inner contexts to
-// be embedded in generated code thus causing leaks and potentially using the
-// wrong native context (i.e. stubs are shared between native contexts).
-class JSContextRelaxation final : public Reducer {
- public:
-  JSContextRelaxation() {}
-  ~JSContextRelaxation() final {}
-
-  Reduction Reduce(Node* node) final;
-};
-
-}  // namespace compiler
-}  // namespace internal
-}  // namespace v8
-
-#endif  // V8_COMPILER_JS_CONTEXT_RELAXATION_H_