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.