Remove NodeRecordTest#unreferencedNodeRecordShouldBeRoot

The test method NodeRecordTest#unreferencedNodeRecordShouldBeRoot is supposed to assert that the node state corresponding to the root of the tree is always a root record. Root records and the root records table were removed from the segment format at r1763136 as part of OAK-2498. As such, NodeRecordTest#unreferencedNodeRecordShouldBeRoot is now useless and can be removed.