Remove unused sysbtn 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_common_config") {
   cflags = []
@@ skipping 1250 matching lines @@
       "xfa/fwl/cfwl_noteloop.cpp",
       "xfa/fwl/cfwl_noteloop.h",
       "xfa/fwl/cfwl_picturebox.cpp",
       "xfa/fwl/cfwl_picturebox.h",
       "xfa/fwl/cfwl_pushbutton.cpp",
       "xfa/fwl/cfwl_pushbutton.h",
       "xfa/fwl/cfwl_scrollbar.cpp",
       "xfa/fwl/cfwl_scrollbar.h",
       "xfa/fwl/cfwl_spinbutton.cpp",
       "xfa/fwl/cfwl_spinbutton.h",
-      "xfa/fwl/cfwl_sysbtn.cpp",
-      "xfa/fwl/cfwl_sysbtn.h",
       "xfa/fwl/cfwl_themebackground.h",
       "xfa/fwl/cfwl_themepart.cpp",
       "xfa/fwl/cfwl_themepart.h",
       "xfa/fwl/cfwl_themetext.h",
       "xfa/fwl/cfwl_timer.cpp",
       "xfa/fwl/cfwl_timer.h",
       "xfa/fwl/cfwl_timerinfo.cpp",
       "xfa/fwl/cfwl_timerinfo.h",
       "xfa/fwl/cfwl_widget.cpp",
       "xfa/fwl/cfwl_widget.h",
@@ skipping 537 matching lines @@
       "//samples",
     ]
   }
   group("fuzzers") {
     testonly = true
     deps = [
       "//testing/libfuzzer",
     ]
   }
 }