Remove /bin/sh call made redundant by hashbang.

[config] / systemfiles / irssi.service

# /etc/systemd/system/irssi.service

[Unit]                                                                          
Description=irssi screen                                                        
                                                                                
[Service]                                                                       
Type=forking                                                                    
User=plom                                                                       
ExecStart=/home/plom/config/bin/screen-irssi.sh
                                                                                
[Install]                                                                       
WantedBy=multi-user.target