doc: Delete info about removed checkbox "Place in system tray"
Summary: Screenshots also need to be updated, but I'm too lazy for that.
Test Plan: none
Reviewers: lueck, broulik, yurchor
Reviewed By: yurchor
Subscribers: plasma-devel, kde-doc-english
Tags: Plasma, Documentation
Differential Revision: https://phabricator.kde.org/D14007