Remove unused FWL Panel code.

BUILD.gn

 # 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.
 
 import("//build_overrides/v8.gni")
 import("//testing/test.gni")
 import("pdfium.gni")
 
 config("pdfium_config") {
   cflags = []
@@ skipping 1025 matching lines @@
       "xfa/fwl/core/cfwl_themepart.h",
       "xfa/fwl/core/cfwl_themetext.h",
       "xfa/fwl/core/cfwl_widgetimpproperties.h",
       "xfa/fwl/core/fwl_appimp.cpp",
       "xfa/fwl/core/fwl_appimp.h",
       "xfa/fwl/core/fwl_error.h",
       "xfa/fwl/core/fwl_formimp.cpp",
       "xfa/fwl/core/fwl_formimp.h",
       "xfa/fwl/core/fwl_noteimp.cpp",
       "xfa/fwl/core/fwl_noteimp.h",
-      "xfa/fwl/core/fwl_panelimp.cpp",
-      "xfa/fwl/core/fwl_panelimp.h",
       "xfa/fwl/core/fwl_sdadapterimp.cpp",
       "xfa/fwl/core/fwl_sdadapterimp.h",
       "xfa/fwl/core/fwl_targetimp.cpp",
       "xfa/fwl/core/fwl_targetimp.h",
       "xfa/fwl/core/fwl_threadimp.cpp",
       "xfa/fwl/core/fwl_threadimp.h",
       "xfa/fwl/core/fwl_timerimp.cpp",
       "xfa/fwl/core/fwl_widgetdef.h",
       "xfa/fwl/core/fwl_widgetimp.cpp",
       "xfa/fwl/core/fwl_widgetimp.h",
       "xfa/fwl/core/fwl_widgetmgrimp.cpp",
       "xfa/fwl/core/fwl_widgetmgrimp.h",
       "xfa/fwl/core/ifwl_adapternative.h",
       "xfa/fwl/core/ifwl_adapterthreadmgr.h",
       "xfa/fwl/core/ifwl_adapterwidgetmgr.h",
       "xfa/fwl/core/ifwl_app.h",
       "xfa/fwl/core/ifwl_dataprovider.h",
       "xfa/fwl/core/ifwl_form.h",
       "xfa/fwl/core/ifwl_notedriver.h",
       "xfa/fwl/core/ifwl_noteloop.h",
-      "xfa/fwl/core/ifwl_panel.h",
       "xfa/fwl/core/ifwl_target.h",
       "xfa/fwl/core/ifwl_themeprovider.h",
       "xfa/fwl/core/ifwl_thread.h",
       "xfa/fwl/core/ifwl_timer.h",
       "xfa/fwl/core/ifwl_tooltiptarget.h",
       "xfa/fwl/core/ifwl_widget.h",
       "xfa/fwl/core/ifwl_widgetdelegate.h",
       "xfa/fwl/core/ifwl_widgetmgr.h",
       "xfa/fwl/core/ifwl_widgetmgrdelegate.h",
       "xfa/fwl/core/include/ifwl_adaptertimermgr.h",
@@ skipping 639 matching lines @@
       "//v8:v8_libplatform",
     ]
     include_dirs += [
       "//v8",
       "//v8/include",
     ]
     configs += [ "//v8:external_startup_data" ]
   }
   configs += [ ":pdfium_config" ]
 }