Deleting VST track instruments

I have eight VST track instruments showing in the list of VST tracks in the rightmost window, even though I have deleted their MIDI tracks.
How do you delete these?

Select No VST instrument for the rack slot.

Yes, all eight are set to No VST Instrument but all eight are still there.

Setting to no instrument leaves the slot in place as long as the track is still in the project window. Removing the track removes the slot too.

So if they are still there you either didn’t delete the track from the project window or maybe you have encountered a bug.

Originally the 8 MIDI tracks used HSO, but I then changed them to use Vienna Symphonic Library.
This is when I realised I couldn’t just delete the track to remove the HSO association, so I created 8 new tracks and copied the MIDI data to them using Alt-drag (perhaps it copied more than the MIDI data?), and I was then able to delete the original tracks. However, the HSO VST track Instruments didn’t go away. I’ve a feeling I’ve seen something about copying MIDI data using different options but can’t find it in the docs.