So, I did some research, and it looks like the updates are stored in: /data/ota-package/. Might not be able to delete the update due to not having the proper permissions. Perhaps, a factory reset is the best way to go in this situation...
EDIT: Hmm, maybe we don't want to factory reset either... if the update hasn't installed already, it's waiting for the user to manually start the update due to automatic updates being disabled. A factory reset may enable automatic updates and starting installing the update... hmmm...
Maybe try clearing the Storage for System Updates (both entries in Settings), DeviceSoftwareOTA, and also for Download Manager.