Remove additional auth.log when enabling logging

[strebitz]

The additional auth.log which is configured when setting sftp_enable_logging = true simply duplicates these syslog facilities: auth,authpriv.* and write the messages to the file /var/log/sftp/auth.log

This duplicates the logs in multiple files and the later one is not managed by logrotate.

As a user of this Ansible role, I want this behaviour removed.