Fix caching issues with related studies. (#1970)

platform/core/src/services/DisplaySetService/DisplaySetService.js

@@ -125,12 +125,6 @@ export default class DisplaySetService {
     }
   };
 
-  hasDisplaySetsForStudy(StudyInstanceUID) {
-    return displaySetCache.some(
-      displaySet => displaySet.StudyInstanceUID === StudyInstanceUID
-    );
-  }
-
   makeDisplaySetForInstances(instances) {
     const instance = instances[0];