You have to purge Pop OS PPA with
sudo apt-get install ppa-purge
sudo ppa-purge ppa:system76/pop
to transform the system back to ordinary Ubuntu. You can use Synaptic to determine left-overs of PopOS and to revert packages to official Ubuntu by specifying their versions.
Manual way is to parse output of saved dpkg -l for pop[0-9] package versions and pop- ...