To remove just libxext6 package itself from Debian Unstable (Sid) execute on terminal:
sudo apt-get remove libxext6
Uninstall libxext6 and it's dependent packages
To remove the libxext6 package and any other dependant package which are no longer needed from Debian Sid.
sudo apt-get remove --auto-remove libxext6
If you also want to delete configuration and/or data files of libxext6 from Debian Sid then this will work:
sudo apt-get purge libxext6
To delete configuration and/or data files of libxext6 and it's dependencies from Debian Sid then execute:
sudo apt-get purge --auto-remove libxext6
More information about apt-get remove
apt-get remove is identical to install except that packages are removed instead of installed. Note that removing a package leaves its configuration files on the system. If a plus sign is appended to the package name (with no intervening space), the identified package will be installed instead of removed.