Use a dedicated method to clear a wizard's session

Simplifies session removal in case a sub-class of Icinga\Web\Wizard
requires special session handling

refs #7163

application/controllers/SetupController.php

@@ -27,7 +27,7 @@ class SetupController extends ActionController
             $installer = $wizard->getInstaller();
             $success = $installer->run();
             if ($success) {
-                $wizard->getSession()->clear();
+                $wizard->clearSession();
             } else {
                 $wizard->setIsFinished(false);
             }

library/Icinga/Web/Wizard.php

@@ -407,6 +407,14 @@ class Wizard
         return Session::getSession()->getNamespace(get_class($this));
     }
 
+    /**
+     * Clear the session being used by this wizard
+     */
+    public function clearSession()
+    {
+        $this->getSession()->clear();
+    }
+
     /**
      * Add buttons to the given page based on its position in the page-chain
      *