Sync the startplasmacompositor with startkde

Summary: startplasma/startkde doesn't use qtpaths anymore.

Test Plan: starts wayland session

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9795