So I downloaded the script and it seems that all it does is insert its own repository URL into your sources list and then uses the apt-get package manager to install it.

To remove, it is really simple, just open up a terminal and type:

sudo apt-get remove heroku-toolbelt

Also, as a word of warning, try not to use scripts written by others to install applications. Some of them can contain extremely dangerous commands which could cause irreversible damage to your data. If you are unsure how to install an application, you can always search Ask Ubuntu for similar questions and answers.