Remove trackExistence check in batch (i.e. always build _exists) data. (#2348)

(cherry picked from commit 5110405f2f)

batch/batch.go

@@ -1313,20 +1313,15 @@ func (b *Batch) makeFragments(frags, clearFrags fragments) (fragments, fragments
 	shardWidth := b.shardWidth()
 	emptyClearRows := make(map[int]uint64)
 
-	// create _exists fragments if needed
-	// TODO(tlt): maybe make this a separate flag for backward compatibility?
-	// (because dax.Table doesn't have this).
-	//if b.index.Options.TrackExistence {
-	if true {
-		var curBM *roaring.Bitmap
-		curShard := ^uint64(0) // impossible sentinel value for shard.
-		for _, col := range b.ids {
-			if col/shardWidth != curShard {
-				curShard = col / shardWidth
-				curBM = frags.GetOrCreate(curShard, "_exists", "")
-			}
-			curBM.DirectAdd(col % shardWidth)
+	// create _exists fragments
+	var curBM *roaring.Bitmap
+	curShard := ^uint64(0) // impossible sentinel value for shard.
+	for _, col := range b.ids {
+		if col/shardWidth != curShard {
+			curShard = col / shardWidth
+			curBM = frags.GetOrCreate(curShard, "_exists", "")
 		}
+		curBM.DirectAdd(col % shardWidth)
 	}
 
 	for i, rowIDs := range b.rowIDs {

importer.go

@@ -66,10 +66,6 @@ func (i *onPremImporter) ImportRoaringShard(ctx context.Context, tid dax.TableID
 	return i.api.ImportRoaringShard(ctx, string(tid), shard, request)
 }
 
-// EncodeImportValues is kind of weird. We're trying to mimic what the client
-// does here (because the Importer interface was originally based off of the
-// client methods). So we end up generating a protobuf-encode byte slice. And we
-// don't really use path.
 func (i *onPremImporter) EncodeImportValues(ctx context.Context, tid dax.TableID, fld *dax.Field, shard uint64, vals []int64, ids []uint64, clear bool) (path string, data []byte, err error) {
 	// This intentionally no-ops. See comment on struct.
 	return "", nil, nil