completely remove mariadb database and root password
You need to delete the data in your datadir, since that is where this is stored. The default location is /var/lib/mysql/. Shut down MariaDB and delete everything inside the datadir without deleting the datadir itself (as you want to avoid having to recreate the directory with exactly the right permissions):
sudo systemctl stop mariadb
sudo rm -rf /var/lib/mysql/*

Then use mysql_install_db to re-initialise the datadir.