displaymanager: Don't run xdg-icon-resource and update-desktop-database.

Don't run xdg-icon-resource and update-desktop-database in the basic
setup for KDM. These tools have absolutely nothing to do with KDM. KDM
does not ship any icons in the hicolor theme, nor a .desktop file (let
alone one that handles any MIME type, the only case in which
update-desktop-database is needed).

I know the original shell script from Manjaro called these when setting
up KDM, but this is really the wrong place. If you need to call these,
they need to go into another module, and you also have to call them
independently of the selected display manager.