Removed category re-appears after restarting qbittorrent

[bhrgunatha]

Please provide the following information

qBittorrent version and Operating System

4.1.5 64bit Arch linux

If on linux, libtorrent and Qt version

Qt: 5.12.0
libtorrent: 1.1.11.0
Boost:1.69

What is the problem

Remove a category that was used as a default category and it re-appears the next time qbittorrent starts.

What is the expected behavior

I expect all removed category not to return.

Steps to reproduce

1. Create a category
2. Set checkbox for Set as default category from the add torrent dialog.
3. Create different categories.
4. Set a different category as the new default category.
5. Remove original default category.
6. Close qbittorent.
7. Original default category re-appears in list of categories.
8. Try to remove category again, It disappears from the list of categories but re-appears when qbittorent is re-started.

Extra info(if any)

I currently have 2 phantom categories that are not used by any torrents that can't be permanently removed. I can remove them and they disappear from the category list, but they both re-appear after re-starting. I've removed other categories that were NOT default categories and they seem to be removed permanently.

[FranciscoPombal]

This is a side effect of leftover fastresume files, which is a separate issue entirely. If you run into the problem of leftover fastresume files in a reproducible way in the latest version, file a new issue report.

As a workaround, remove the fastresumes of the corresponding torrents that have those categories from the BT_backup folder.

Duplicate of #7242