Address a comment.

Author: kazutakahirata

llvm/include/llvm/ADT/DirectedGraph.h

@@ -181,10 +181,6 @@ template <class NodeType, class EdgeType> class DirectedGraph {
 
   DirectedGraph() = default;
   explicit DirectedGraph(NodeType &N) : Nodes() { addNode(N); }
-  DirectedGraph(const DGraphType &G) = default;
-  DirectedGraph(DGraphType &&RHS) = default;
-  DGraphType &operator=(const DGraphType &G) = default;
-  DGraphType &operator=(DGraphType &&G) = default;
 
   const_iterator begin() const { return Nodes.begin(); }
   const_iterator end() const { return Nodes.end(); }