cmGraphEdge: remove custom copy ctor

Also, merge the other ctors using default args.

Source/cmGraphAdjacencyList.h

@@ -15,26 +15,11 @@
 class cmGraphEdge
 {
 public:
-  cmGraphEdge()
-    : Dest(0)
-    , Strong(true)
-  {
-  }
-  cmGraphEdge(int n)
-    : Dest(n)
-    , Strong(true)
-  {
-  }
-  cmGraphEdge(int n, bool s)
+  cmGraphEdge(int n = 0, bool s = true)
     : Dest(n)
     , Strong(s)
   {
   }
-  cmGraphEdge(cmGraphEdge const& r)
-    : Dest(r.Dest)
-    , Strong(r.Strong)
-  {
-  }
   operator int() const { return this->Dest; }
 
   bool IsStrong() const { return this->Strong; }