To uninstall xhtml-relaxng just follow these instructions.
sudo apt-get remove xhtml-relaxng
This will remove just the xhtml-relaxng package itself.
sudo apt-get remove --auto-remove xhtml-relaxng
This will remove the xhtml-relaxng package and any other dependant packages which are no longer needed.
If you also want to delete your local/config files for xhtml-relaxng then this will work.
sudo apt-get purge xhtml-relaxng
Or similarly, like this xhtml-relaxng
sudo apt-get purge --auto-remove xhtml-relaxng
|Maintainer||Debian XML/SGML Group <email@example.com>|
|Description||Implementation in RELAX NG of the modularization of XHTML|