remove unused option --logfile in mx initscript

if --syslog is specified, --logfile option in twistd is not used. I think we should remove the --logfile option (or maybe having some bool value that would allow to switch between syslog and user-defined logfile).

(from redmine: created on 2015-04-13, closed on 2015-11-19, relates #6925 (closed))