kcms/ksmserver: remove system restart message

Marco Martinrequested to merge
work/cherry-pick-966d0a95 into Plasma/6.3

Now the session save behavior change works immediately, remove the restart system message

CCBUG:449077

(cherry picked from commit 966d0a95)

966d0a95 kcms/ksmserver: remove system restart message

Co-authored-by: Marco Martin notmart@gmail.com

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