image: remove leftover LCOW platform checks

This removes some of the checks that were added in 0cba7740d4,
but should no longer be needed.

- `image/store.Delete()`: no need to validate image platform to delete it.
- `image/tarexporter/takeLayerReference()`: use `image.OperatingSystem()` and
  fail early to prevent constructing the `ChainID()`.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

image/store.go

@@ -229,13 +229,10 @@ func (is *store) Delete(id ID) ([]layer.Metadata, error) {
 	if imageMeta == nil {
 		return nil, fmt.Errorf("unrecognized image ID %s", id.String())
 	}
-	img, err := is.Get(id)
+	_, err := is.Get(id)
 	if err != nil {
 		return nil, fmt.Errorf("unrecognized image %s, %v", id.String(), err)
 	}
-	if !system.IsOSSupported(img.OperatingSystem()) {
-		return nil, fmt.Errorf("unsupported image operating system %q", img.OperatingSystem())
-	}
 	for id := range imageMeta.children {
 		is.fs.DeleteMetadata(id.Digest(), "parent")
 	}

image/tarexport/save.go

@@ -7,7 +7,6 @@ import (
 	"os"
 	"path"
 	"path/filepath"
-	"runtime"
 	"time"
 
 	"github.com/docker/distribution"
@@ -149,18 +148,14 @@ func (l *tarexporter) takeLayerReference(id image.ID, imgDescr *imageDescriptor)
 	if err != nil {
 		return err
 	}
+	if os := img.OperatingSystem(); !system.IsOSSupported(os) {
+		return fmt.Errorf("os %q is not supported", os)
+	}
 	imgDescr.image = img
 	topLayerID := img.RootFS.ChainID()
 	if topLayerID == "" {
 		return nil
 	}
-	os := img.OS
-	if os == "" {
-		os = runtime.GOOS
-	}
-	if !system.IsOSSupported(os) {
-		return fmt.Errorf("os %q is not supported", os)
-	}
 	layer, err := l.lss.Get(topLayerID)
 	if err != nil {
 		return err