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