Always allow clearing download queue

It's possible that the queue only contains a failed invalid (does not exist anymore in the database) chapter which then causes graphql errors.
In this case the queue was considered empty which prevented it from getting cleared.
Without being able to clear it, the error could not be fixed.

src/screens/DownloadQueue.tsx

@@ -138,7 +138,7 @@ export const DownloadQueue: React.FC = () => {
         setAction(
             <>
                 <Tooltip title={t('download.queue.label.delete_all')}>
-                    <IconButton onClick={clearQueue} size="large" disabled={isQueueEmpty}>
+                    <IconButton onClick={clearQueue} size="large">
                         <DeleteSweepIcon />
                     </IconButton>
                 </Tooltip>