To uninstall xdm just follow these instructions.
sudo apt-get remove xdm
This will remove just the xdm package itself.
sudo apt-get remove --auto-remove xdm
This will remove the xdm package and any other dependant packages which are no longer needed.
If you also want to delete your local/config files for xdm then this will work.
sudo apt-get purge xdm
Or similarly, like this xdm
sudo apt-get purge --auto-remove xdm
|Maintainer||Debian X Strike Force <firstname.lastname@example.org>|
|Description||X display manager remote machines. It provides services similar to those provided by init, getty, and login on character-based terminals: prompting for login name and password, authenticating the user, and running a session. xdm supports XDMCP|