Uninstall composer
To remove just composer package itself from Ubuntu 16.04 (Xenial Xerus) execute on terminal:
sudo apt-get remove composer
Uninstall composer and it's dependent packages
To remove the composer package and any other dependant package which are no longer needed from Ubuntu Xenial.
sudo apt-get remove --auto-remove composer
Purging composer
If you also want to delete configuration and/or data files of composer from Ubuntu Xenial then this will work:
sudo apt-get purge composer
To delete configuration and/or data files of composer and it's dependencies from Ubuntu Xenial then execute:
sudo apt-get purge --auto-remove composer
https://www.howtoinstall.co/en/ubuntu/xenial/composer?action=remove