Fix some recursive undo removal

~StatefulDiffCommand() may trigger UndoTransaction::command_death()
which may delete the StatefulDiffCommand() that's just being destroyed.

This depends on the signal-connection order, which is undefined.
In any case when a shared_ptr<> object is being destroyed it means
that all references to it are already gone. There's no need to
emit drop_references from the d'tor.

libs/pbd/stateful_diff_command.cc

@@ -68,8 +68,6 @@ StatefulDiffCommand::StatefulDiffCommand (boost::shared_ptr<StatefulDestructible
 
 StatefulDiffCommand::~StatefulDiffCommand ()
 {
-        drop_references ();
-
         delete _changes;
 }