properly log output of willie in systemd

with the --quiet flag, the verbose core config setting has zero effect
and there would be no way for admins to diagnose bot startup problems.

removing the flag has no harmful effects: willie's output is dutifully
logged to systemd's journal files, according to the proper log levels
defined in the config file. since willie doesn't support syslog
logging (see #658 for that), this is pretty much the *only* way to
have logs of willie's activities and problems outside of IRC.