`diffoci rm` does not collect garbage blobs when running with `local` backend

[AkihiroSuda]

diffoci/cmd/diffoci/commands/remove/remove.go

Lines 21 to 23 in 63094dc

	BUG: when using the "local" backend, the remove command
	does not remove blobs associated with the image.
	(Workaround: remove the cache directory periodically by yourself.)

GC here does not seem working as expected

diffoci/cmd/diffoci/commands/remove/remove.go

Lines 55 to 57 in 63094dc

	if gcErr := backend.MaybeGC(ctx); gcErr != nil {
	log.G(ctx).WithError(gcErr).Warn("Failed to do GC")
	}