Don't register a new service for screenshots

Summary:
If you're going to have kwin in the service name anyway there's
absolutely no benefit to having clients not use the existing
org.kde.KWin.

Test Plan: Object is still registered.

Reviewers: Plasma, graesslin

Reviewed By: Plasma, graesslin

Subscribers: graesslin, plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D3357