Fix PreviewJob memory leak

setAutoDelete(false) means that it won't be deleted when
merely doing kill() and forgetting the pointer (so manual "delete" in
this file won't happen).

Found by looking for code that uses KJob::setAutoDelete(false)

src/project/dialogs/slideshowclip.cpp

@@ -203,6 +203,7 @@ void SlideshowClip::slotEnableThumbs(int state)
         if (m_thumbJob) {
             disconnect(m_thumbJob, &KIO::PreviewJob::gotPreview, this, &SlideshowClip::slotSetPixmap);
             m_thumbJob->kill();
+            m_thumbJob->deleteLater();
             m_thumbJob = nullptr;
         }
     }