In an openstack environment with ceph backend you could let cinder flatten the volume automatically during creation, that way the volume becomes independent from the snapshot which can be deleted afterwards. The cinder config is this one, and you would need to set it to true:

# Flatten volumes created from snapshots to remove dependency from volume to
# snapshot (boolean value)
#rbd_flatten_volume_from_snapshot = false

I don't know how that works with other cinder backends, though.