remove DestroyEdge

This special edge type is no longer used. While we still have the option
of encoding more meaning into the edged themselves, having one special
edge type used only in one specific case was easily overlooked, as
dag.BasicEdge is assumed in all other cases.

terraform/edge_destroy.go

@@ -1,17 +0,0 @@
-package terraform
-
-import (
-	"fmt"
-
-	"github.com/hashicorp/terraform/dag"
-)
-
-// DestroyEdge is an edge that represents a standard "destroy" relationship:
-// Target depends on Source because Source is destroying.
-type DestroyEdge struct {
-	S, T dag.Vertex
-}
-
-func (e *DestroyEdge) Hashcode() interface{} { return fmt.Sprintf("%p-%p", e.S, e.T) }
-func (e *DestroyEdge) Source() dag.Vertex    { return e.S }
-func (e *DestroyEdge) Target() dag.Vertex    { return e.T }