Deleting the reduction amount/percentage or start/end dates does not remove a discount on a product.
To delete a discount price, it is necessary to delete the correponding line from the Specific Prices panel.
You can select multiple products to display the assocated prices you need to delete.
To remove discounts in bulk, use the interface dedicated to Specific Prices (accessible from Catalog menu).
Use the filtering tools (if necessary) to then select all specific prices displayed in the panel using the mass-selection icon .
And finally click on the delete icon to definitely remove the selected specific prices.