do not explicitly check NotShownIn it's not necessary and incomplete

Summary:
KService::noDisplay() includes qualification for

• NoDisplay entry itself
• OnlyShowIn and NotShowIn
• X-KDE-OnlyShowOnQtPlatforms
• authorization WRT KCMs

as such checking nodisplay is good enough to disqualify services in all
relevant scenarios

Test Plan: - read the code of noDisplay()

Reviewers: broulik

Reviewed By: broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3705