Deprecate DerivableSchemaNode

This interface's sole method, getOriginal(), is used only by Binding runtime components. Those should be able to cope without it, as mdsal-binding-generator has enough knowledge to replace its use. Supporting this method costs us in all SchemaTreeEffectiveStatements, to a varying degree – for example, ContainerEffectiveStatementImpl is larger by 0/8/8/0 bytes because of it.