Remove transactions.oldestOpenUnpreparedReadTimestamp from serverStatus (see https://docs.mongodb.com/master/reference/command/serverStatus/#serverstatus.transactions.oldestOpenUnpreparedReadTimestamp)
The serverStatus metric oldestOpenUnpreparedReadTimestamp is named to represent the oldest read timestamp of any unprepared transaction. However, as described in
WT-4154, this value is the oldest_active_reader, which is the minimum read timestamp of all active readers. Timestamped reads can occur without multi-document transactions, e.g. all secondary reads, so this value exists even when the server has no open, unprepared transactions. We will remove this serverStatus metrics, and the oldest active read timestamp will be added to WT statistics in WT-4704.