I found this old thread discussing the same question:
https://www.odoo.com/forum/help-1/question/hide-discounts-on-order-lines-on-quotations-invoices-16510
However, the solution suggested is outdated and doesn't apply to the new QWeb-based reporting engine. I just wondered, if there is a better/easier solution by now to automatically determine, whether the "discount" column should be displayed or not. Also, it would be nice to hide the discount in an invoice line, if there is none given or it is negative.