I want to disable admin approval notification when user is getting registered to a drupal site. I don't want to send notification mail to the admin.
You can disable the administration approval option in configuration page /admin/config/people/accounts of your site.
Otherwise, if you wish to maintain adminstration approval but only disable the email, add the following line at your settings.php:
$conf['user_mail_register_no_approval_required_notify'] = FALSE;