Cannot delete fully index entry if content file doesn't exist.

[RustyNova016]

If we try to remove an index entry or content entry when it's already deleted, cacache throw an io::ErrorKind::NotFound. This is fine, but when using RemoveOpts and remove_fully, a missing content file will throw an early error and prevent the deletion of the bucket.

A solution would be to simply ignore NotFound errors, as it is already in the intended final state.