fix: username sanitized twice in UserAuth::authUser()

Remove usage of filter_var on username in UserAuth::authUser() as
it's already done before.

core/App/UserAuth.php

@@ -115,12 +115,7 @@ public function checkSchema(): void
      */
     public function authUser(string $username, string $password): string
     {
-        // TODO: FILTER_SANITIZE_STRING is deprecated as of PHP 8.1, replace by htmlspecialchars() instead
-
-        // Sanitize username
-        $username = filter_var($username, FILTER_SANITIZE_STRING, array( 'flags' => FILTER_FLAG_STRIP_LOW | FILTER_FLAG_STRIP_HIGH));
         $username = trim($username, ' ');
-
         $user = $this->userTable->findByName($username);
 
         if ($user) {