kvserver: remove priority reset during setProcessing

Previously, a replicaItem’s priority was cleared when marked as processing, to
indicate it was no longer in the priority queue. This behavior made sense when
the purgatory queue did not track priorities. However, we now need to preserve
priorities for items in purgatory as well since they will be calling into
baseQueue.processReplica. This commit removes the priority reset in
replicaItem.SetProcessing(), ensuring that the enqueue time priority is retained
when replicas are popped from the heap and passed into the purgatory queue
properly. No behavior change should happen from this change.

Author: wenyihu6

pkg/kv/kvserver/queue.go

@@ -137,7 +137,6 @@ type replicaItem struct {
 
 // setProcessing moves the item from an enqueued state to a processing state.
 func (i *replicaItem) setProcessing() {
-	i.priority = 0
 	if i.index >= 0 {
 		log.Fatalf(context.Background(),
 			"r%d marked as processing but appears in prioQ", i.rangeID,