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

components/pdf/browser/BUILD.gn

 # 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.
 
 import("//build/config/features.gni")
 
 static_library("browser") {
   sources = [
-    "open_pdf_in_reader_prompt_client.h",
     "pdf_web_contents_helper.cc",
     "pdf_web_contents_helper.h",
     "pdf_web_contents_helper_client.h",
   ]
 
   deps = [
     "//base",
     "//components/pdf/common",
     "//content/public/browser",
     "//ipc",
   ]
 }