python: remove unused buffer

[syundo0730]

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Description

In the Python implementation of ChunkBuilder.reset(), the self.buffer object is recreated but never actually referenced. This PR removes the unused self.buffer variable to simplify the code.

I manually ran the following examples without encountering any errors:

python3 python/examples/jsonschema/pointcloud_csv_to_mcap.py python/examples/jsonschema/bus.csv
python3 python/examples/protobuf/image_example.py ptoro_image.mcap
python3 python/examples/protobuf/writer.py proto.mcap
python3 python/examples/raw/writer.py raw.mcap

Additionally, I ran:

mcap doctor out.mcap
mcap doctor ptoro_image.mcap
mcap doctor proto.mcap
mcap doctor raw.mcap

All checks passed without issues.

[indirectlylit]

this looks correct - thank you!