I just updated to ver 2.80.1, as a bit of added information. That box was already unchecked in preferences > Look & Feel, off by default as you said, and I had never turned it on. It was still showing the same icon in the systemtray. I clicked on the little gold icon in the lower right corner for the KDE [plasma desktop (I think)] Panel Tool Box. As I swept my mouse cursor over the Calibre Icon in the System Tray, a little pop-up box rose up, and there was an X in the upper right corner like for closing a window. I clicked on it, and the Calibre icon disappeared from the System Tray. Don't know how it got in there in the first place, but that seems to have gotten rid of it and solved the problem. If the problem shows up again later, I will post again. Thank you for your prompt reply and your assistance on this matter.
P.S. Thank you for producing a wonderful piece of software, I find it highly useful, well thought out & arranged, and easy to use when I do need to make use of it.