If you don't want to use a category  you can delete the category like this:

• iOS, Android and Windows 11 / Amazon FireOS:
  Settings > Categories & Rules > swipe left on the category > delete
  or
  Select the main- or Sub Category > 'Edit' > scroll down to 'Delete Category'
• macOS:
  Settings > Categories & Rules > Right click on the category > Delete 

Important:
If the Main Category does not contain Subcategories, the category will be deleted and the transactions, that belonged to the category will be uncategorized.
If the Main Category contains one or more Subcategories, you will be asked if you want to keep the Subcategories (then they will become to Main Categories) or if you want to delete also the Subcategories (then the transaction of the Subcategories will be uncategorized).