[Unit]
Description=rTorrent
Requires=network.target local-fs.target

[Service]
Type=forking
RemainAfterExit=yes
KillMode=none
User=rtorrent
ExecStart=/bin/rm -f /home/rtorrent/.session/*.lock
WorkingDirectory=/home/rtorrent/

[Install]
WantedBy=multi-user.target

$ll /home/rtorrent/.session/*.lock
-r--r--r-- 1 rtorrent www-data 16 Sep 21 17:23 /home/rtorrent/.session/rtorrent.lock