Remove outdated note about autostart not working

Summary:
Just tested, worked fine. That line has been there since 5.0 days, I do
vaguely recall fixing this at some point.

CCBUG: 401399

Reviewers: #plasma

Subscribers: plasma-devel, kde-doc-english

Tags: #plasma, #documentation

Differential Revision: https://phabricator.kde.org/D17709