Terminology fixes in KCMs: KDE-as-the-desktop -> Plasma

Summary: Fix the reference to KDE (or remove them when not strictly needed).

Test Plan: The code still compiles.

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: sebas, lukas, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6469