kv: delete some paranoia

Before this patch, Transaction.Clone() was not making a copy of an
empty, but with capacity, splice. This seems dangerous to me.
There was also some paranoia about it in the DistSender that can be
removed if cloning does the right thing.

pkg/kv/dist_sender.go

@@ -530,12 +530,7 @@ func (ds *DistSender) initAndVerifyBatch(
 				//
 				// We already have a clone of our txn (see above), so we can
 				// modify it freely.
-				//
-				// Zero the existing data. That makes sure that if we had
-				// something of size zero but with capacity, we don't re-use the
-				// existing space (which others may also use). This is just to
-				// satisfy paranoia/OCD and not expected to matter in practice.
-				ba.Txn.ResetObservedTimestamps()
+
 				// OrigTimestamp is the HLC timestamp at which the Txn started, so
 				// this effectively means no more uncertainty on this node.
 				ba.Txn.UpdateObservedTimestamp(nDesc.NodeID, ba.Txn.OrigTimestamp)

pkg/roachpb/data.go

@@ -715,7 +715,7 @@ func (t Transaction) LastActive() hlc.Timestamp {
 // keys with the intent spans.
 func (t Transaction) Clone() Transaction {
 	mt := t.ObservedTimestamps
-	if len(mt) != 0 {
+	if mt != nil {
 		t.ObservedTimestamps = make([]ObservedTimestamp, len(mt))
 		copy(t.ObservedTimestamps, mt)
 	}