monitoring/commands: Fix `ToggleFeatureCommandForm::createElements()'

`Icinga\Web\Form::createElements()' no longer requires to return an array of elements
because elements should be created directly.

refs #6593

modules/monitoring/application/forms/Command/Instance/ToggleFeatureCommandForm.php

@@ -63,8 +63,7 @@ abstract class ToggleFeatureCommandForm extends CommandForm
                 ->getQuery()
                 ->fetchRow();
         }
-        return array(
+        $this->addElement(
-            $this->createElement(
             'checkbox',
             $this->feature,
             array(
@@ -72,8 +71,8 @@ abstract class ToggleFeatureCommandForm extends CommandForm
                 'autosubmit'    => $this->inline,
                 'value'         => $enabled
             )
-            )
         );
+        return $this;
     }
 
     /**