remove unused import and fix phpdocs (#22833: Short-term admin accounts)

Author: lfolco

app/code/Magento/Security/Model/Plugin/AdminUserForm.php

@@ -7,8 +7,6 @@
 
 namespace Magento\Security\Model\Plugin;
 
-use Magento\Security\Model\UserExpiration;
-
 /**
  * Add the `expires_at` form field to the User main form.
  *
@@ -99,7 +97,7 @@ public function aroundGetFormHtml(
     /**
      * Loads a user expiration record by user ID.
      *
-     * @param $userId string
+     * @param string $userId
      * @return string
      */
     private function loadUserExpirationByUserId($userId)