Revert everything related to Damien's work in 2008 (he will commit a new version soon).

Here are the reverted patches:
8c0d1003b0,
25a3114287,
9afbaf6342,
dc0005f4e1,
543190f2bc.

src/ltlvisit/destroy.cc

@@ -25,10 +25,17 @@ namespace spot
 {
   namespace ltl
   {
-    void
-    destroy_visitor::doit_default(formula* c)
+    namespace
     {
-      formula::unref(c);
+      class destroy_visitor: public postfix_visitor
+      {
+      public:
+	virtual void
+	doit_default(formula* c)
+	{
+	  formula::unref(c);
+	}
+      };
     }
 
     void