Revert "display-manager: fix argument handling of sddm"

This reverts commit 6b7c5ba5.

Unfortunately it seems like this broke slim, lightdm and gdm (see #25068
and #23264). This is already reverted in the 17.03 branch (99dfb6d).

TODO: We need tests for slim and lightdm and fix the test for gdm
(failing since 2016-10-26) to prevent such breakage in the future.