Just delete the entry in COGI.
As far as the delivery note is concerned you need to talk with your end users or business experts why that field is mandatory and is it supposed to be a mandatory field for just purchasing goods receipt or for both purchasing and production good receipt.
It may be possible that the serial number profile has been removed from material master before any serial number has been assigned to any of the stock. (Serial number profile cannot be removed if serial numbers have ever been assigned to that material)