Remove nullable referenceSetId in SearchReferences.
Remove nullable referenceSetId in SearchReferences.

diekhans/ga4gh-schemas,buske/schemas,dglazer/schemas,Jeltje/ga4gh-schemas,buske/schemas,ga4gh/ga4gh-schemas,dnanexus-rnd/schemas,macieksmuga/schemas,heuermh/ga4gh-schemas,diekhans/ga4gh-schemas,benedictpaten/schemas,ga4gh/schemas,hjellinek/schemas,benedictpaten/schemas,dnanexus-rnd/schemas,saupchurch/schemas,pcingola/schemas,Jeltje/ga4gh-schemas,hjellinek/schemas,dcolligan/schemas