Fix duplicate crate references in SUMMARY.md

Remove duplicate references to polars and serde crates from the Data Engineering section.
These crates were already listed in the "Crates hay dùng" section, causing mdbook build to fail with:

"Duplicate file in SUMMARY.md: ./crates/polars.md"
"Duplicate file in SUMMARY.md: ./crates/serde.md"
etc.

The data-processing.md page already describes these crates, so the duplicate
sub-items in the table of contents were unnecessary.

Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: claude

src/SUMMARY.md

@@ -174,12 +174,6 @@
 # Data Engineering
 
 - [Xử lý và phân tích dữ liệu](./data-engineering/data-processing.md)
-  - [Polars](./crates/polars.md)
-  - [serde](./crates/serde.md)
-    - [serde_json](./crates/serde/serde_json.md)
-    - [serde_toml](./crates/serde/serde_toml.md)
-    - [serde_csv](./crates/serde/serde_csv.md)
-    - [serde_yaml](./crates/serde/serde_yaml.md)
 - [High-performance data pipeline](./data-engineering/first-data-pipeline.md)
 - [Building scalable data-driven applications using Rust](./data-engineering/data-driven.md)
 - [Rust as an alternative to Python for data engineering tasks](./data-engineering/rust-as-alternative-python.md)