Refer: https://askubuntu.com/questions/681317/how-to-purge-steam-ppa-completely
You can manually remove steam related PPA file from /etc/apt/sources.list.d/ and related gpgkey from /etc/apt/trusted.gpg.d/
Open a terminal
sudo ls -ltr /etc/apt/sources.list.d/
sudo ls -ltr /etc/apt/trusted.gpg.d/Remove from the above anything that matches the application you are trying to purge