Remove references for .subject in ObservationLab and ObservationClinicalTest

FHIR-37459 and FHIR-37505 changes included a request to add ObservationLab.subject and Observation.ClinicalTest.subject references to include Device and Location. The US Core .subject references are limited to Patient as all of US Core is based on patient data. We do not have good use cases for lab or clinical test observations about a device or location. Also, these added references create errors in the QA report for QI-Core and would require a variance from US Core that has insufficient justification at this time. Recommend remove the references to Device and Location in ObservationClinicalTest and ObservationLab, retaining only the reference for Patient.