The 'simple' categories used in Advanced Module Manager are not actual separate categorie items.
It is just a field that contains the name of a 'category'.
The category field simply shows all names that are being used.
If you remove the category from all the items using that same category name, it will no longer show.

Try removing your deleted items.

Can you give me (super) admin and (s)ftp access so I can take a look?
You can use the 'Confidential information' button in the forum editor to hide sensitive information.