Remove unused Loggable{Batch}::arrow_field

[teh-cmc]

It doesn't make any sense for a ComponentBatch to have any say in what the final ArrowField should look like.

An ArrowField is a Chunk/RecordBatch/Schema-level concern that only makes sense during IO/transport/FFI/storage/etc, and which requires external context that a single ComponentBatch on its own has no idea of.

---

Part of a lot of clean up I want to while we head towards:

• Eager/early serialize of components to arrow in Rust and C++ #7245
• Migrate from re_arrow2 to arrow #3741

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[teh-cmc]

@rerun-bot full-check

[github-actions]

Started a full build: https://github.com/rerun-io/rerun/actions/runs/10525350667

[teh-cmc]

@rerun-bot full-check

[github-actions]

Started a full build: https://github.com/rerun-io/rerun/actions/runs/10525995551