nixos/tests/lightdm: Drop "session" screenshot.

Using waitForWindow on the IceWM root window doesn't necessarily mean
that the panel will be shown. In the lightdm test, we only make sure
that the login is working and thus it doesn't matter how the session
itself will look or whether IceWM is broken, so we don't need that
screenshot.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>