Remove if statement for deleting parents in box

[jmshrv]

https://github.com/UnicornsOnLSD/finamp/blob/07c833e2f28b3073cc0f110f56c76ac5b567c3f8/lib/services/DownloadsHelper.dart#L132

Since DownloadedParents is now used purely to store downloaded parents, we don't have to make this check anymore when deleting a parent.

This check actually causes a bug where a parent may not be deleted by the box.

To reproduce:

1. Download a playlist
2. Download an album containing a song from that playlist
3. Try deleting the album. It will not be correctly marked as deleted since the DownloadedParent still exists.

On another note, we probably don't even have to delete downloadedChildren from the DownloadedParent since a DownloadedParent will always have all of its children.