To uninstall openmpt123 just follow these instructions.
sudo apt-get remove openmpt123
This will remove just the openmpt123 package itself.
sudo apt-get remove --auto-remove openmpt123
This will remove the openmpt123 package and any other dependant packages which are no longer needed.
If you also want to delete your local/config files for openmpt123 then this will work.
sudo apt-get purge openmpt123
Or similarly, like this openmpt123
sudo apt-get purge --auto-remove openmpt123
|Maintainer||Debian Multimedia Maintainers <firstname.lastname@example.org>|
|Description||module music library based on OpenMPT -- music player|