In order to delete an extracting rule go to extracting rules by clicking on 'Extracting Rules' button of the relevant extractor. Then hover by your mouse over the extracting rule you want to delete and you will see that 'Delete' button will appear. Click on it and then confirm the warning.
Note that if you delete an extracting rule, then extracted data related to it will not be available anymore. Keep in mind that this operation cannot be undone! However, you can always create the same one later at any time.