Copilot: Take advantage of IOptionPage::setWidgetCreator()

Less boilerplate for the implementation add user code access to
IOptionPage::{apply,finish} is planned to be removed.

Change-Id: I03bbde071fdf673bee0568baa9ae9fab8d816590
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/copilot/copilotplugin.cpp

@@ -50,7 +50,7 @@ void CopilotPlugin::initialize()
 
 void CopilotPlugin::extensionsInitialized()
 {
-    CopilotOptionsPage::instance().init();
+    (void)CopilotOptionsPage::instance();
 }
 
 void CopilotPlugin::restartClient()