systemd: Drop obsolete compatibility flag

In v6 [1], the compatibility mode has been removed and -c is a basically
a no-op. So just drop that obsolete command line flag.

[1] http://www.rsyslog.com/doc/v6compatibility.html

rsyslog.service.in

@@ -3,7 +3,7 @@ Description=System Logging Service
 
 [Service]
 ExecStartPre=/bin/systemctl stop systemd-kmsg-syslogd.service
-ExecStart=@sbindir@/rsyslogd -n -c6
+ExecStart=@sbindir@/rsyslogd -n
 Sockets=syslog.socket
 StandardOutput=null