Remove useless code for Segment V2

For segment v2, ```RowBlock2``` 's ```_delete_state``` is useless, ```Delete Logic```  actually works in ```SegmentIterator::next_batch```;
In ```BetaRowsetReader::next_block```,  when ```RowBlockV2::convert_to_row_block``` is called, RowBlockV2's  ```_delete_state``` ignored.
So I think this logic can be removed from ```column_reader```/```segment_iterator```/```generic_iterators.cpp```.
