In my case, I have no logger entry in configuration.yaml. Nevertheless, there is a home-assistant.log file and it’s about 82 Kb. Whatever I see in there is also found in /var/log/syslog.
The home-assistant.log file is automatically purged after every restart whereas the duplicated entries in syslog are retained.
Anyway, that’s how it works on my system (version 0.89 in a venv).