fix(ui): clean up theme selector and migration for single-theme setup

- Remove dead code in migrateOldTheme() that set Theme parameter for
  KindredCreate.yaml, which no longer exists in the Stylesheets build
- Remove orphaned Classic.yaml and KindredCreate.yaml parameter files
  from Stylesheets CMakeLists and from disk
- Remove unused includes in ThemeSelectorWidget.cpp (QApplication,
  QEvent, Gui/Application.h)

src/Mod/Start/Gui/ThemeSelectorWidget.cpp

@@ -21,8 +21,6 @@
  *                                                                          *
  ***************************************************************************/
 
-#include <QApplication>
-#include <QEvent>
 #include <QHBoxLayout>
 #include <QLabel>
 #include <QString>
@@ -30,7 +28,6 @@
 
 #include "ThemeSelectorWidget.h"
 #include <gsl/pointers>
-#include <Gui/Application.h>
 #include <Gui/PreferencePackManager.h>
 
 using namespace StartGui;