Revert "feat(datafusion): expose PartitionKeysKind getter on IcebergTableScan (#21)"

[toutane]

This reverts commit c10a519e4d8801d21aae3e6b9dda8a901fc0d4c8.

PartitionKeysKind was previously used to expose the partitioning strategy derived from a table's partition spec, allowing callers such as df-executor to configure partitioning at runtime. This approach had two fundamental issues: it bypassed DataFusion's native configuration API, and it attempted to control partitioning logic during execution - too late in the lifecycle, since partitioning is already determined at planning time.

To address this, https://github.com/toutane/iceberg-rust/pull/7/changes introduces IcebergConfig, which implements DataFusion's ConfigExtension interface. This replaces the former PartitionKeysKind-based mechanism with a first-class integration into DataFusion's configuration system.

[datadog-official]

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