You can uninstall or removes an installed festival-hi package itself from Ubuntu 16.04 LTS (Xenial Xerus) through the terminal,
$ sudo apt-get remove festival-hi
If you would like to remove festival-hi and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove festival-hi
If you use with purge options to festival-hi package all the configuration and dependent packages will be removed.
$ sudo apt-get purge festival-hi
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 festival-hi