Remove dead code since PDFWebContentsHelper::ShowOpenInReaderPrompt isn't called anymore.

components/pdf/browser/open_pdf_in_reader_prompt_client.h

-// Copyright (c) 2012 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 COMPONENTS_PDF_BROWSER_OPEN_PDF_IN_READER_PROMPT_CLIENT_H_
-#define COMPONENTS_PDF_BROWSER_OPEN_PDF_IN_READER_PROMPT_CLIENT_H_
-
-#include "base/strings/string16.h"
-
-namespace content {
-struct LoadCommittedDetails;
-}
-
-namespace pdf {
-
-class OpenPDFInReaderPromptClient {
- public:
-  virtual ~OpenPDFInReaderPromptClient() {}
-
-  virtual base::string16 GetMessageText() const = 0;
-
-  virtual base::string16 GetAcceptButtonText() const = 0;
-
-  virtual base::string16 GetCancelButtonText() const = 0;
-
-  virtual bool ShouldExpire(
-      const content::LoadCommittedDetails& details) const = 0;
-
-  virtual void Accept() = 0;
-
-  virtual void Cancel() = 0;
-};
-
-}  // namespace pdf
-
-#endif  // COMPONENTS_PDF_BROWSER_OPEN_PDF_IN_READER_PROMPT_CLIENT_H_