Have user-manager write to SDDM's new config file

Summary:
user-manager still wants to write to sddm.conf and it's using sddm_kcm's function to do so, but this function no longer supports writing to sddm.conf

That means auto-login settings are no longer getting saved anywhere.

This patch tweaks user-manager to write to /etc/sddm.conf.d/kde_setings.conf, making the feature work again.

Path specifying is removed on the user-manager side for writing, SDDM's auth helper does that now.

BUG: 342722
FIXED-IN: 5.18.1

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26813