It is kind of non intuitive. You have to actually Edit the morph itself and change the Slider path, which is = to custom category.
If there is only one slider in custom category, then the category would be renamed. And if you clear the path, the custom category would be removed and slider moved into the root.
If there is more than one slider, then renaming the path, would create a new category to which slider would be moved. To get rid of category, you'd need to rename a path for each slider it contains.
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