Coding policy fixes.

Using "this->" violates coding policy section 2.7.

common/dialogs/panel_setup_severities.cpp

@@ -1,7 +1,7 @@
 /*
  * This program source code file is part of KiCad, a free EDA CAD application.
  *
- * Copyright (C) 2020 KiCad Developers, see AUTHORS.txt for contributors.
+ * Copyright (C) 2020-2021 KiCad Developers, see AUTHORS.txt for contributors.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
@@ -141,8 +141,8 @@ PANEL_SETUP_SEVERITIES::PANEL_SETUP_SEVERITIES( PAGED_DIALOG* aParent,
     gridSizer->Fit( scrollWin );
     panelSizer->Add( scrollWin, 1, wxEXPAND | wxALL, 5 );
 
-    this->SetSizer( panelSizer );
-    this->Layout();
+    SetSizer( panelSizer );
+    Layout();
     panelSizer->Fit( this );
 }