If you do not want a hyperlink category to be available for selection anymore, but you can not delete it, since it is or has been used, it is possible to hide the hyperlink category.
This will result in the hyperlink category remaining in the settings, however, it cannot be chosen, when uploading links under resources.
This is how you hide hyperlink categories
Choose Settings 
Go to resource categories
Thereafter, click on Hide hyperlink category in the functions-column of the hyperlink category 
Click on confirm to save the changes
The hyperlink category is now hidden and can not be used to upload files
If you do not want the hyperlink category to be shown in the list, turn off display hidden items in the list under Filter
To use this hyperlink category again, click on Show hyperlink category 
