Hi Michal,
Sometimes it’s a good idea to ‘refresh’ your portfolio. I’ve deleted a couple of tracks myself that never sold anything and I will do it again soon. Music changes really fast and stuff will sound dated at some point in time. If a track doesn’t sell it could have millions of different reasons, but most of the times it will be because apparently people aren’t interested in it, which is not a shame, it just happens. Sometimes a track gets noticed all of a sudden and sell better from that point on, happened to my second track, no sales for about a year and a half, and now 9… Whether you choose to delete it or not is up to you!
Good luck!