Fix double free of undo commands (amend 9e6435ff14)

This fixes a case when deleting a plugin, deletes all automation
undo/redo events:
  <UndoTransaction name="add automation event">
    <MementoCommand type-name="ARDOUR::AutomationList">
   ...


`delete this;` calls the d'tor which emits drop_references(),
that leads to UndoTransaction::command_death() destroying the
object, whichh causes a double free.

libs/pbd/pbd/memento_command.h

@@ -127,7 +127,8 @@ public:
 	}
 
 	void binder_dying () {
-		delete this;
+		/* delegate to UndoTransaction::command_death */
+		drop_references ();
 	}
 
 	void operator() () {