Previous event attributes persist/cannot be deleted

Hi,

I had created a set of attributes in the past. I have now created a different one and deleted the old one on wp-admin (setting > general options). Both attributes were created using the following syntax (#_ATT{Difficulty}{Beginner|Intermediate|Advanced|Any Level}
).

I am now seeing the new attributed drop-down on the event-creation form, but i am also seeing the old one. How could i remove the old attributes?

Many thanks,
Yvan

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