You can uninstall or removes an installed dhcp3-relay package itself from Debian 7 (Wheezy) through the terminal,
$ sudo apt-get remove dhcp3-relay
If you would like to remove dhcp3-relay and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove dhcp3-relay
If you use with purge options to dhcp3-relay package all the configuration and dependent packages will be removed.
$ sudo apt-get purge dhcp3-relay
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 dhcp3-relay