[#4211] Retain delete markers on index tables only during backfill

Summary:
Fix negation during ShouldRetainDeleteMarkers
Retain delete markers on index tables only during backfill

Issue introduced by 7c46f82

Test Plan: TBD. add unit test

Reviewers: bogdan, mikhail

Reviewed By: mikhail

Subscribers: kannan, ybase

Differential Revision: https://phabricator.dev.yugabyte.com/D8282

src/yb/tablet/tablet_retention_policy.cc

@@ -108,7 +108,7 @@ void TabletRetentionPolicy::UnregisterReaderTimestamp(HybridTime timestamp) {
 bool TabletRetentionPolicy::ShouldRetainDeleteMarkersInMajorCompaction() const {
   // If the index table is in the process of being backfilled, then we
   // want to retain delete markers until the backfill process is complete.
-  return !metadata_.schema().table_properties().IsBackfilling();
+  return metadata_.schema().table_properties().IsBackfilling();
 }
 
 HybridTime TabletRetentionPolicy::HistoryCutoffToPropagate(HybridTime last_write_ht) {