Depend on KIO::PreviewJob::defaultPlugins() and KF 5.40

Summary: master now has KF 5.40, so require that and always use KIO::PreviewJob::defaultPlugins(), which lets us get rid of some unnecessary code

Test Plan: Tested in KDE Neon. Compiles and deploys fine.

Reviewers: #frameworks, broulik, elvisangelaccio

Reviewed By: elvisangelaccio

Differential Revision: https://phabricator.kde.org/D8847