code: use macros for default node functions

Easier to maintain

app/node/math/merge/merge.cpp

@@ -38,11 +38,6 @@ MergeNode::MergeNode()
   SetFlags(kDontShowInParamView);
 }
 
-Node *MergeNode::copy() const
-{
-  return new MergeNode();
-}
-
 QString MergeNode::Name() const
 {
   return tr("Merge");

app/node/math/merge/merge.h

@@ -31,9 +31,7 @@ class MergeNode : public Node
 public:
   MergeNode();
 
-  NODE_DEFAULT_DESTRUCTOR(MergeNode)
-
-  virtual Node* copy() const override;
+  NODE_DEFAULT_FUNCTIONS(MergeNode)
 
   virtual QString Name() const override;
   virtual QString id() const override;