I just started to wonder if there’s any solution to this, found some old topics about some sysex files that could sort the used and unused kits, but for older OS … etc.
Is there any other newer and simpler form of getting rid of the unused kits, they just occupy useless space anyways? (seems a bit weird not to have this by default out of the box - “delete unused kits”, not even with overbridge?? why not a kit manager?? c’mon, we’re moving to Mars very soon…) 
You can’t actually delete kits, you can only clear them.
You may already know that unused kits are marked with a hash in the kit browser, unfortunately you have to load a kit before you can clear it and you have to save a kit before you can rename it.
It would be nice if OB were extended to include kit manipulation.
“LOAD KIT” MENU > HIGHLIGHT ANY KIT > FUNC + CLEAR = the kit is gone from the kit list.
The same is with copy and paste (between projects also).
There are 128 kits in the kit list, nothing you do will change that.
Clearing a kit simply sets the sound and FX parameters, for that kit, to their default values.
What i just said earlier, leaves an empty space in the kits list 
. Permanent. So, if you had 100 kits in the project but want for some reason to clear the space, you can leave only 10 if you want. It’s not the “clear kit” thing. That will just leave your kit in the list, of course empty, but still present. I needed to delete them, not to clear (empty) them, but as in the performance menu, it’s not at all clear when and where that copy/paste/delete thing works. I couldn’t immagine that i can copy my entire performance setup just by hitting copy on the “performance configure” line. It’s not that intuitive I couldn’t immagine if i am inside the “load kit” or “save kit” list, that i can simply copy/paste/clear all the list, cause i was in a loading menu or in a saving menu, not just browsing in a kit mannager. Well … it’s ok as long as it works
Load an empty kit and use it to overwrite the kit you want to delete…