You can uninstall or removes an installed happy package itself from Debian 8 (Jessie) through the terminal,
$ sudo apt-get remove happy
If you would like to remove happy and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove happy
If you use with purge options to happy package all the configuration and dependent packages will be removed.
$ sudo apt-get purge happy
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 happy
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