You can uninstall or removes an installed agenda.app package itself from Ubuntu 16.04 LTS (Xenial Xerus) through the terminal,
$ sudo apt-get remove agenda.app
If you would like to remove agenda.app and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove agenda.app
If you use with purge options to agenda.app package all the configuration and dependent packages will be removed.
$ sudo apt-get purge agenda.app
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove agenda.app