I am sorry but I don't see the reason for that.
That part is never used because the previous if clause:

        if ($sOneTimePassword!='' && Yii::app()->getConfig("use_one_time_passwords") && md5($sOneTimePassword)==$user->one_time_pw)
        {
            $user->one_time_pw='';
            $user->save();
            $this->id = $user->uid;
            $this->user = $user;
            $this->errorCode = self::ERROR_NONE;
        }

If you need to comment the following part then you are doing something wrong.

I made a mistake, it isn't application/controllers/UserIdentity.php but application/core/UserIdentity.php,
Sorry.

I understand where is my problem :

LSYii_Application Object
(
[config:protected] => Array
(
...
[use_one_time_passwords] =>
...

While in my file / config.php :

...
// Debug Settings
$debug = 0;

$use_one_time_passwords = true;

I am looking why the variable is not true just before if ($sOneTimePassword!='' && Yii::app()->getConfig("use_one_time_passwords") && md5($sOneTimePassword)==$user->one_time_pw)

I found it!

I add "'use_one_time_passwords'=>true" at the end of /application/config/config.php like it :

            'debug'=>0,
            'debugsql'=>0, // Set this to 1 to enanble sql logging, only active when debug = 2
            'use_one_time_passwords'=>true
    )

);

and it work.

Thank you for your help