Add value sets and remove description from ResearchStudy relatesTo extension

Will change as suggested in comments and add an attribute       relatedArtifact 0..* RelatedArtifact https://jira.hl7.org/browse/FHIR-32973  describes changes made to RelatesTo type which meet the original reasons for adopting the relatesTo extension. There is already an attribute relatesTo in ResearchStudy so the action required is to remove the relatesTo extension.  However we know of more than one user who has adopted this extension, we will therefore add a note and a point to the URL of a version where the extension existed.