If you are billing clients with only flat amounts and don't need to use the "Units" column, you may be interested in hiding it.
This will change your invoices so there is only one editable amount per line item though, so it will become less flexible in that way. It will, however, simplify your invoices so it's less confusing if you happen to always bill flat amounts.
If you'd like to try out this option, you can update it in your Invoice Settings.
Note: This change will apply to all of your past and future invoices, though it will not alter the total amounts of any invoices you have created in the past.