reduceThreadUpdates used to return both an updated threadStore object and corresponding threadStoreOperations. reduceThreadUpdates has a single callsite and that's in reduceThreadStore where only the threadStoreOperations property of the returned object was being pulled out anyways... so in this diff we remove the updated object to clean things up.

(Since this function is no longer really a "reducer," we'll rename this to something like generateOpsForThreadUpdates)

Depends on D4196

assertThreadStoreThreadsAreEqual hasn't flagged anything since it's been included in staff builds so we can assume "the ops are good."