If the Converted value in the Resolution field is renamed or deleted, it is readded back into the list of values automatically at a later time.
This is working as designed. When a defect is converted into a user story, the resolution of the defect is set to Converted. If the value does not exist in the field definition, the system automatically adds the value back into the Resolution field.
It is best to leave this value as is since it is required by the conversion process