`updateSystemMetadata()` does not remove obsoletes field

[rogerdahl]

When doing a read-modify-write operation on System Metadata in which the obsoletes field is removed, the obsoletes field remains in GMN. Need to check the behavior for all optional fields.

Example received by getSystemMetadata():

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ns3:systemMetadata xmlns:ns2="http://ns.dataone.org/service/types/v1" xmlns:ns3="http://ns.dataone.org/service/types/v2.0">
    <serialVersion>1</serialVersion>
    <identifier>10.0309_rwstg1k34w_201981021525</identifier>
    <formatId>http://www.isotc211.org/2005/gmd</formatId>
    <size>4475</size>
    <checksum algorithm="MD5">8d236914945bb03ed9211aabe463568e</checksum>
    <submitter>CN=urn:node:mnTestRW,DC=dataone,DC=org</submitter>
    <rightsHolder>CN=urn:node:mnTestRW,DC=dataone,DC=org</rightsHolder>
    <accessPolicy>
        <allow>
            <subject>public</subject>
            <permission>read</permission>
        </allow>
    </accessPolicy>
    <replicationPolicy numberReplicas="3" replicationAllowed="true"/>
    <obsoletes>10.0309_rwstg1k34u_2019810194555</obsoletes>
    <archived>false</archived>
    <dateUploaded>2019-09-10T21:52:05.571+00:00</dateUploaded>
    <dateSysMetadataModified>2019-09-10T22:22:37.661+00:00</dateSysMetadataModified>
    <originMemberNode>urn:node:mnTestRW</originMemberNode>
    <authoritativeMemberNode>urn:node:mnTestRW</authoritativeMemberNode>
    <seriesId>10.0309/rwstg1k34w</seriesId>
    <fileName>10.0309_rwstg1k34w_201981021525.xml</fileName>
</ns3:systemMetadata>

Example submitted with updateSystemMetadata().

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ns3:systemMetadata xmlns:ns2="http://ns.dataone.org/service/types/v1" xmlns:ns3="http://ns.dataone.org/service/types/v2.0">
    <serialVersion>1</serialVersion>
    <identifier>10.0309_rwstg1k34w_201981021525</identifier>
    <formatId>http://www.isotc211.org/2005/gmd</formatId>
    <size>4475</size>
    <checksum algorithm="MD5">8d236914945bb03ed9211aabe463568e</checksum>
    <submitter>CN=urn:node:mnTestRW,DC=dataone,DC=org</submitter>
    <rightsHolder>CN=urn:node:mnTestRW,DC=dataone,DC=org</rightsHolder>
    <accessPolicy>
        <allow>
            <subject>public</subject>
            <permission>read</permission>
        </allow>
    </accessPolicy>
    <replicationPolicy numberReplicas="3" replicationAllowed="true"/>
    <archived>false</archived>
    <dateUploaded>2019-09-10T21:52:05.571+00:00</dateUploaded>
    <dateSysMetadataModified>2019-09-10T22:22:37.661+00:00</dateSysMetadataModified>
    <originMemberNode>urn:node:mnTestRW</originMemberNode>
    <authoritativeMemberNode>urn:node:mnTestRW</authoritativeMemberNode>
    <seriesId>10.0309/rwstg1k34w</seriesId>
    <fileName>10.0309_rwstg1k34w_201981021525.xml</fileName>
</ns3:systemMetadata>