[chore](be) Refine tablet reader context and generic reader handling

be/src/format/generic_reader.h

@@ -261,10 +261,6 @@ class GenericReader : public ProfileCollector {
 
     const size_t _MIN_BATCH_SIZE = 4064; // 4094 - 32(padding)
 
-    // never let batch size be 0 because _do_get_next_block uses it as the
-    // upper bound of a `while (block->rows() < batch_size)` loop and a 0 would make the reader
-    // return without setting eof, causing the scanner to spin on empty blocks.
-    const size_t _DEFAULT_BATCH_SIZE = 4064; // 4094 - 32(padding)
     TPushAggOp::type _push_down_agg_type {};
 
 public:

be/src/storage/rowset/rowset_reader_context.h

@@ -76,9 +76,6 @@ struct RowsetReaderContext {
     // Effective adaptive batch size byte budget. 0 means disabled internally.
     size_t preferred_block_size_bytes = 8388608UL;
 
-    // Points to the "true" output column list before non-direct-mode expansion.
-    // Used by BlockReader to map expanded storage columns back to the requested output layout.
-    const std::vector<ColumnId>* origin_return_columns = nullptr;
     bool is_unique = false;
     //record row num merged in generic iterator
     uint64_t* merged_rows = nullptr;

be/src/storage/tablet/tablet_reader.cpp

@@ -201,10 +201,6 @@ Status TabletReader::_capture_rs_readers(const ReaderParams& read_params) {
     // Propagate general read limit for DUP_KEYS and UNIQUE_KEYS with MOW
     _reader_context.general_read_limit = read_params.general_read_limit;
 
-    // Preserve the original requested output layout so BlockReader can map expanded storage
-    // columns (for non-direct AGG/UNIQUE paths) back to the final output block.
-    _reader_context.origin_return_columns = read_params.origin_return_columns;
-
     return Status::OK();
 }