Once you’re on 2.1.0p15 (or possibly earlier), you no longer need check_mk.socket (or other names along the same lines).
For anyone wondering by, I believe the full solution to problems when upgrading the agent from 2.0.0 to 2.1.0 is:
apt remove check-mk-agent # Remove 2.0.xxxx
systemctl stop check_mk.socket # May not exist, but if it does, get rid of it
systemctl disable check_mk.socket # ...and make sure it doesn't come back again after a reboot
apt update
apt install check-mk-agent # with the intention of install 2.1.xxxx!
You’ll also need to do something like cmk-agent-ctl register ... to switch your agent into SSL mode (and not legacy mode), but that’s a different subject.