prune: no longer disable automatic index updates

this allows prune to resume an interrupted prune run.

internal/repository/packer_manager.go

@@ -200,8 +200,5 @@ func (r *Repository) savePacker(ctx context.Context, t restic.BlobType, p *packe
 	r.idx.StorePack(id, p.Packer.Blobs())
 
 	// Save index if full
-	if r.noAutoIndexUpdate {
-		return nil
-	}
 	return r.idx.SaveFullIndex(ctx, r)
 }