As mentionned, left-click on the rack name, then select “No VST instrument” … it will disappear from the rack. Make sure you have no MIDI track referring to this VSTi.
Is there a way to automatically remove unused rack instruments? Thank you
No.
Regards. 
Thank you very much for this step-by-step clarification. I spent a couple of hours on it and couldn’t find a way to remove it. Now it’s clear. 
Hey all! i ran into this problem with the Cubase 11 Pro Trial today…somehow when i added a new instrument track- it added another HALion SE track? which cannot be deleted. but , as outlined here, F11 opens up the rack, and from there you select the instrument in question- (i actually cannot reproduce it again now haha) but, no matter what i tried the VSTi wouldn’t delete - until i ‘DOUBLE-CLICKED’ on the ‘No VST Instrument’ at top of the drop-list! Give it a try! 
then it deletes and you can then remove the leftover folders in the track list. (im on PC for what it’s worth.)
This one did the trick. (Besides the name of the vst there is also a drop down with the choice ‘no vst instument’, but that’s not the one, it’s the name of the rack instrument!
Wow, just add a right click and delete option. Cubase menus are sucking
Thank you - that was the observation I needed.
you are a life saver! Thank you sir
I think every new user has to ask this question. It’s very non-intuitive, especially coming from other DAWs.
I finally found it. I had to enable the right hand side menu to see it. It is NOT in the left side in the rack!
