Don't use a custom signal at all

QThread::finished should be enough in all the cases.

YACReaderLibrary/library_creator.h

@@ -62,7 +62,6 @@ private:
     void cleanup(QSqlDatabase &db, const QString &target);
 
 signals:
-    void completed();
     void coverExtracted(QString);
     void folderUpdated(QString);
     void comicAdded(QString, QString);