storpool driver: drop the image-to-volume-and-back overridden methods

In some situations, the logic in the "let's create an image from a Cinder volume and vice versa" methods that the StorPool driver overrides is outdated: it does not always work correctly with StorPool-Cinder-backed Glance images, and it can sometimes do too much work in other cases. It would be best if the copy_volume_to_image() and copy_image_to_volume() methods of the StorPool driver are dropped outright, since Cinder has, in the meantime, learned to do both of these operations correctly.