Change the retired_by value for Reference App Metadata concepts from 1 to null

The changes need to happen in this module: https://github.com/openmrs/openmrs-module-referencemetadata for these xml files: https://github.com/openmrs/openmrs-module-referencemetadata/tree/master/api/src/main/resources

In the XML files, the concepts have retired_by=1, yet retired="false" as in the example below:

<concept_class concept_class_id="24" name="Pharmacologic Drug Class" description="Class of medications based on pharmacologic properties as opposed to therapeutic properties" creator="1" date_created="2014-02-07 22:16:02.0" retired="false" retired_by="1" uuid="b4535251-9183-4175-959e-9ee67dc71e78"/>