To completely remove phpmyadmin on ubuntu or linux mint, You can execute the below commands from terminal (consol-Konsol)
and walk with the instructions come our one by one, But pay attention to the Password, You have to type it twice.
sudo apt-get purge libapache2-mod-php5
sudo apt-get install libapache2-mod-php5
sudo apt-get purge phpmyadmin
sudo apt-get install phpmyadmin
sudo /etc/init.d/apache2 restart
Also you can check out the Digital ocean post in same topic here