Remove checkpoint methods from builder backend

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

builder/builder.go

@@ -130,13 +130,6 @@ type Backend interface {
 	// ContainerUpdateCmdOnBuild updates container.Path and container.Args
 	ContainerUpdateCmdOnBuild(containerID string, cmd []string) error
 
-	// CheckpointCreate checkpoints a running container
-	CheckpointCreate(container string, config types.CheckpointCreateOptions) error
-	// CheckpointDelete deletes a container's checkpoint
-	CheckpointDelete(container string, checkpoint string) error
-	// CheckpointList lists the available checkpoints for a container
-	CheckpointList(container string) ([]types.Checkpoint, error)
-
 	// ContainerCopy copies/extracts a source FileInfo to a destination path inside a container
 	// specified by a container object.
 	// TODO: make an Extract method instead of passing `decompress`