Switch to ImageServer.UntagImage in RemoveImage handler

Add an UntagImage() method to pkg/storage/ImageServer, which will check
if the passed-in NameOrID is a name.  If so, it merely removes that name
from the image, removing the image only if it was the last name that the
image had.  If the NameOrID is an image ID, the image is removed, as
RemoveImage() does.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

server/image_remove.go

@@ -40,7 +40,7 @@ func (s *Server) RemoveImage(ctx context.Context, req *pb.RemoveImageRequest) (r
 		}
 	}
 	for _, img := range images {
-		err = s.StorageImageServer().RemoveImage(s.ImageContext(), img)
+		err = s.StorageImageServer().UntagImage(s.ImageContext(), img)
 		if err != nil {
 			logrus.Debugf("error deleting image %s: %v", img, err)
 			continue