Deleting is irreversible, and we cannot restore permanently deleted items. Confirmation is always required before any deletion, so be sure you're not accidentally deleting the wrong item.
Delete an issue category
Select Issues in the sidebar or select it in More.
Select Categories at the top of the page.
Click on the right-hand side of the category and select Delete.
In the pop-up window, click Delete category.

Deleting an issue category doesn't affect issues already reported under that category.