Remove fpm_logfile from logrotate script

The default location of `fpm_logfile` on all operating systems is inside `fpm_log_dir`. This causes logrotate to emit a warning every time:

    /etc/cron.daily/logrotate:
    error: php5-fpm:1 duplicate log entry for /var/log/php5-fpm/fpm-master.log
    error: found error in /var/log/php5-fpm/fpm-master.log /var/log/php5-fpm/*.log , skipping