A good solvent-based adhesive remover should be able to take those marks off. I can recommend De-solv-it "sticky stuff remover" but I have no idea if that's available everywhere. It has a silly name but it works very well.
Actually there isn't any residues left on the surface, it's perfectly smooth without any asperities. I wonder if it has been caused by the effect of the sun? Because the marks really looks like decals now...
Those markings are caused by uneven sun bleaching due to the stickers blocking U/V radiation to varying degrees of effectiveness.
So they can’t be removed, right?
Unfortunately, no. It's UV damage to the shell that would normally be even across the entire surface (and thus not really noticeable).