To remove just mythes-de-ch package itself from Debian 9 (Stretch) execute on terminal:
sudo apt-get remove mythes-de-ch
Uninstall mythes-de-ch and it's dependent packages
To remove the mythes-de-ch package and any other dependant package which are no longer needed from Debian Stretch.
sudo apt-get remove --auto-remove mythes-de-ch
If you also want to delete configuration and/or data files of mythes-de-ch from Debian Stretch then this will work:
sudo apt-get purge mythes-de-ch
To delete configuration and/or data files of mythes-de-ch and it's dependencies from Debian Stretch then execute:
sudo apt-get purge --auto-remove mythes-de-ch
More information about apt-get remove
apt-get remove is identical to install except that packages are removed instead of installed. Note that removing a package leaves its configuration files on the system. If a plus sign is appended to the package name (with no intervening space), the identified package will be installed instead of removed.