remove root output eval nodes from destroy

If we're adding a node to remove a root output from the state, the
output itself does not need to be re-evaluated. The exception for root
outputs caused them to be missed when we refactored resource destruction
to only use the existing state.

terraform/graph_builder_apply.go

@@ -165,7 +165,7 @@ func (b *ApplyGraphBuilder) Steps() []GraphTransformer {
 		// directly.  A destroy is identical to a normal apply, except for the
 		// fact that we also have configuration to evaluate. While the rest of
 		// the unused nodes can be programmatically pruned (via
-		// pruneUnusedNodesTransformer), root module outputs only have an
+		// pruneUnusedNodesTransformer), root module outputs always have an
 		// implied dependency on remote state. This means that if they exist in
 		// the configuration, the only signal to remove them is via the destroy
 		// command itself.

terraform/transform_destroy_edge.go

@@ -207,10 +207,10 @@ func (t *pruneUnusedNodesTransformer) Transform(g *Graph) error {
 		modules = append(modules, mod)
 	}
 
-	// Sort them by path length, longest first, so that start with the deepest
+	// Sort them by path length, longest first, so that we start with the
-	// modules.  The order of modules at the same tree level doesn't matter, we
+	// deepest modules. The order of modules at the same tree level doesn't
-	// just need to ensure that child modules are processed before parent
+	// matter, we just need to ensure that child modules are processed before
-	// modules.
+	// parent modules.
 	sort.Slice(modules, func(i, j int) bool {
 		return len(modules[i].addr) > len(modules[j].addr)
 	})

terraform/transform_output.go

@@ -88,13 +88,14 @@ func (t *destroyRootOutputTransformer) Transform(g *Graph) error {
 
 		deps := g.UpEdges(v)
 
-		// the destroy node must depend on the eval node
-		deps.Add(v)
-
 		for _, d := range deps {
 			log.Printf("[TRACE] %s depends on %s", node.Name(), dag.VertexName(d))
 			g.Connect(dag.BasicEdge(node, d))
 		}
+
+		// We no longer need the expand node, since we intend to remove this
+		// output from the state.
+		g.Remove(v)
 	}
 	return nil
 }