Golint fix with ro_layer.go

A small golint fix with ro_layer.go.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

layer/ro_layer.go

@@ -142,11 +142,7 @@ func storeLayer(tx MetadataTransaction, layer *roLayer) error {
 			return err
 		}
 	}
-	if err := tx.setOS(layer.layerStore.os); err != nil {
-		return err
-	}
-
-	return nil
+	return tx.setOS(layer.layerStore.os)
 }
 
 func newVerifiedReadCloser(rc io.ReadCloser, dgst digest.Digest) (io.ReadCloser, error) {