I assume that you don't need MySQL anymore and you have already taken
backup of your important databases. I suggest you to take backup of
your important MySQL databases.
Note: Your all mysql data will be lost.
First make sure that MySQL service is stopped.
sudo systemctl stop mysql
Remove MySQL related all packages completely.
sudo apt-get purge mysql-server mysql-client mysql-common mysql-server-core-* mysql-client-core-*
Remove MySQL configuration and data. If you have changed database location in your MySQL configuration, you need to replace /var/lib/mysql according to it.
sudo rm -rf /etc/mysql /var/lib/mysql
(Optional) Remove unnecessary packages.
sudo apt autoremove
(Optional) Remove apt cache.
sudo apt autoclean