User wants to edit the Manual Invoice template and remove the Hours column from it.
You can do that by editing the Manual Invoice template in Global Settings > Invoice Templates. From the Show drop-down, select Manual Invoice and click Detail for the invoice template on the grid that you want to edit. In the Edit Template screen, under Layout > Time tab, you can un-check Hours and save it as a new template that does not display the hours. Check Core Help for details on editing invoice templates.