Python: Use setup function for remaining plugin items

... and remove the now-empty plugin pimpl.

Change-Id: I0db27757e832a0e3ac52b938a4d6ad820adfc195
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/python/pythonsettings.h

@@ -71,4 +71,6 @@ private:
     static void saveSettings();
 };
 
+void setupPythonSettings(QObject *guard);
+
 } // Python::Internal