Fix filtering skill up doc (openvinotoolkit#1238)

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### Summary

- There was a mistake and this PR fixes it by removing duplicated
`filter_func()`.

### How to test
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### Checklist
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- [ ] I have added unit tests to cover my changes.​
- [ ] I have added integration tests to cover my changes.​
- [ ] I have added the description of my changes into
[CHANGELOG](https://github.com/openvinotoolkit/datumaro/blob/develop/CHANGELOG.md).​
- [ ] I have updated the
[documentation](https://github.com/openvinotoolkit/datumaro/tree/develop/docs)
accordingly

### License

- [ ] I submit _my code changes_ under the same [MIT
License](https://github.com/openvinotoolkit/datumaro/blob/develop/LICENSE)
that covers the project.
  Feel free to contact the maintainers if that's a concern.
- [ ] I have updated the license header for each file (see an example
below).

```python
# Copyright (C) 2023 Intel Corporation
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Signed-off-by: Kim, Vinnam <[email protected]>

docs/source/docs/level-up/intermediate_skills/09_data_filtering.rst

@@ -106,10 +106,6 @@ The more advanced Python example is given :doc:`this notebook <../../jupyter_not
                 # Accept Bboxes smaller than 50% of the image size
                 return bbox_size < 0.5 * image_size
 
-            def filter_func(item: DatasetItem) -> bool:
-                h, w = item.media_as(Image).size
-                return h > 1024 or w > 1024
-
             dataset_path = '/path/to/data'
             dataset = Dataset.import_from(dataset_path, format='datumaro')