Remove kolibri-desktop-auth-plugin

[dylanmccall]

This application currently includes kolibri-desktop-auth-plugin v1.0.0: https://github.com/learningequality/kolibri-installer-gnome/blob/master/build-aux/flatpak/modules/python3-kolibri-desktop-auth-plugin.json.

This plugin exists only to provide a migration from its old DesktopUser model to using the OSUser model which is built in to Kolibri. Once we are satisfied that this migration has been around for long enough, we should remove the plugin completely.