KIKIMR-19139 Bugfix iterator double return

ydb/core/tablet_flat/flat_part_iter_multi.h

@@ -656,8 +656,6 @@ namespace NTable {
             // Save an estimate for MaxVersion
             MaxVersion = rowVersion;
             return EReady::Data;
-
-            return Exhausted();
         }
 
         using TPartGroupRowIt::IsValid;