In Key Editor, it’s not possible to delete a Note with a double click on it, with any Tool at all.
And you can’t disable the “double click opens note expression” behaviour.
I too miss the feature that Object Selection Tool or Draw Tool can delete Notes by a double click.
Or by a single click while holding down some modifier keys, for example SHIFT+ALT.
Too bad there isn’t a Cubase Feature Request section here on the forum (AFAIK).
Kind Regards