I think you need to edit ...\Silent Hunter 4\Data\Menu\cfg\OrdersBar.cfg.
Scroll down to 'Station3' and remove buttons 1 and 3. If you want to get rid of the raise/lower buttons too, remove buttons 2 and 4 also. You'll probably then have to rename the remaining buttons so they start from 1.
I've not tested this but I've added buttons to the HUD by doing the opposite, so you should be OK

It goes without saying to backup the files first.