storage: remove redundant calls to UpdateObservedTimestamp

85d4627 added a new update to a transaction's observed timestamps.
This update to the observed timestamps made the other two in the
deferred function immediately above it redundant.

This commit removes those redundant updates and removes the proto
clones we were performing to permit those updates.

Release note: None

pkg/storage/store.go

@@ -2781,18 +2781,13 @@ func (s *Store) Send(
 			// this node.
 			if pErr != nil {
 				pErr.OriginNode = ba.Replica.NodeID
-				if txn := pErr.GetTxn(); txn != nil {
-					// Clone the txn, as we'll modify it.
-					pErr.SetTxn(txn)
-				} else {
+				if txn := pErr.GetTxn(); txn == nil {
 					pErr.SetTxn(ba.Txn)
 				}
-				pErr.GetTxn().UpdateObservedTimestamp(ba.Replica.NodeID, now)
 			} else {
 				if br.Txn == nil {
 					br.Txn = ba.Txn
 				}
-				br.Txn.UpdateObservedTimestamp(ba.Replica.NodeID, now)
 				// Update our clock with the outgoing response txn timestamp
 				// (if timestamp has been forwarded).
 				if ba.Timestamp.Less(br.Txn.Timestamp) {