[opt](Variant) avoid unnecessary mem for variant extracted columns (a…

…pache#43567)

_field_name_to_index and _field_id_to_index is unnecessary for variant
subcolumns, since they use column path as identifier

be/src/olap/tablet_schema.cpp

@@ -1000,6 +1000,7 @@ void TabletSchema::init_from_pb(const TabletSchemaPB& schema, bool ignore_extrac
         if (column->is_variant_type()) {
             ++_num_variant_columns;
         }
+
         _cols.emplace_back(std::move(column));
         if (!_cols.back()->is_extracted_column()) {
             _field_name_to_index.emplace(StringRef(_cols.back()->name()), _num_columns);