Remove unused FWL Panel code.

xfa.gyp

 {
   "variables": {
     "pdf_enable_v8%": 1,
   },
   "target_defaults": {
     "defines": [
       "PDF_ENABLE_XFA",
     ],
     'msvs_disabled_warnings': [
       4005, 4018, 4146, 4333, 4345, 4267,
@@ skipping 170 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 549 matching lines @@
           "sources": [],
         }],
         ["OS == 'mac'", {
           "configurations": {},
           "sources": [],
         }],
       ]
     }
   ]
 }