[Nova] We are dropping the 'delete_instance_files' virt driver interface

Hi all especially maintainers of out-of-tree drivers, Please pay attention to this change. https://review.opendev.org/#/c/714653/ We are dropping 'delete_instance_files' virt driver interface, and will use "cleanup_instance" to take charge of lingering instance cleanup, including instance files deleting and whatever we add in the future. Best Regards, Luyao