Revert "Fix saveRoot() to delete root node from recentDB"

This reverts commit 1735fa9.

Deleting root node from recentDB doesn't help in terms of performance or
memory.

nodedb.go

@@ -767,11 +767,9 @@ func (ndb *nodeDB) saveRoot(hash []byte, version int64, flushToDisk bool) error
 
 	key := ndb.rootKey(version)
 	ndb.updateLatestVersion(version)
+	ndb.recentBatch.Set(key, hash)
 	if flushToDisk {
 		ndb.snapshotBatch.Set(key, hash)
-		ndb.recentBatch.Delete(key)
-	} else {
-		ndb.recentBatch.Set(key, hash)
 	}
 
 	return nil