Fix variable shadowing causing LayersSize to be reported as 0

Signed-off-by: Cezar Sa Espinola <cezarsa@gmail.com>

daemon/disk_usage.go

@@ -99,7 +99,6 @@ func (daemon *Daemon) SystemDiskUsage(ctx context.Context) (*types.DiskUsage, er
 	for platform := range daemon.stores {
 		layerRefs := daemon.getLayerRefs(platform)
 		allLayers := daemon.stores[platform].layerStore.Map()
-		var allLayersSize int64
 		for _, l := range allLayers {
 			select {
 			case <-ctx.Done():