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