I don’t think there’s a way to remove all VST “Instrument Tracks” at once from the rack. If this is confirmed, I’d highly suggest to @Steinberg_Archived to add this feature.
Also, removing an instrument is tedious, because it needs to scroll the instrument’s list all the way up to reach, at the top of the list, “No instrument”. It’d need a simple “Delete Instrument” for each single instrument instance.
Thank you, best regards.