In SERVER-47690, we added an "atClusterTime" field to cursor replies for non-transaction snapshot reads:

> db.runCommand({find: "collection", readConcern: {level: "snapshot"}})
// Server selects a read timestamp and replies with it
{
  ok: 1,
  cursor: {
    firstBatch: [...],
    id: ...,
    atClusterTime: Timestamp(...)
  }
}

As a consequence of the implementation, cursor replies to snapshot reads that are in transactions also gained "atClusterTime", and we decided that was fine. Let's change our minds and omit the field for replies to commands in transactions. It's not useful.