Core: Provide empty implementation for IOptionPageWidget::finish()

Having it pure virtual only lead to a lot of unnecessary
re-implementation.

Change-Id: I37f71d1a160e21b93c7c73a6304b0eb6d5f0441d
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/systemsettings.cpp

@@ -157,7 +157,6 @@ public:
 
 private:
     void apply() final;
-    void finish() final {}
 
     void showHelpForFileBrowser();
     void resetFileBrowser();