As you already know how you installed it and what version you're working with, you can do the following:
Stop the MySQL service:
sudo systemctl stop mysqld
Purge MySQL, databases, and configurations:
sudo apt purge mysql-server mysql-common mysql-server-core-* mysql-client-core-*
Remove any additional database files:
sudo rm -rf /var/lib/mysql/
The folder where the configuration was and any stranglers:
sudo rm -rf /etc/mysql/
Clean the logs:
sudo rm -rf /var/log/mysql
Delete the user-generated during installation:
sudo deluser --remove-home mysql
Finally, get rid of the usergroup that was created during installation:
sudo delgroup mysql
That should get rid of everything. If you installed a third-party PPA in order to install MySQL then you'll need to remove that.
sudo add-apt-repository --remove ppa:theppayouused/ppa
And that should be everything related to MySQL and nothing else.