When rmi localfs/oss/s3 backend nydus image, the blob cache will not be deleted

[Zephyrcf]

When using nydusify to convert an OCI image to Nydus format, if the backend type (--backend-type) is set to localfs, oss, or s3, the digest information of blobs will be missing in the generated image manifest file.

This does not affect the operation of the container, but it will cause a problem: when the user executes rmi to delete this Nydus image, the corresponding blob cache file cannot be removed from the storage backend. These blob files that have not been cleaned will continue to occupy storage space, causing a waste of storage resources.

[Zephyrcf]

In this way, the corresponding blob can be found.

[imeoer]

Storing blobs in the external backend has indeed introduced many new issues, so perhaps we should consider removing the support of external backend.