kcms/ksmserver: remove "Offer shutdown options" setting

This setting's text implies that it disables the logout screen or removes all the other options from it, but it does not. In fact it appears to do nothing. No session-related UI that I or anyone else can find respects the value set here, making it a setting that seems from the user's perspective to be completely useless.

Let's remove it.

BUG: 431573 FIXED-IN: 5.27

@teams/vdg @teams/usability @davidedmundson @davidre

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