Remove the unused KafkaProducer.produce batch path

[lukashes]

KafkaProducer.produce (src/sink/kafka/producer.zig:230) batch-produces via rd_kafka_produce_batch, but nothing in the pipeline calls it. The only callers are in src/benchmarks/kafka_bench.zig, and the doc comment already flags it as "Reserved for future optimizations". The live path is send (one message at a time), used by the processor.

sendMessage (:286) is pub too, but its only production caller is send in the same file (tests and benches call it directly). It could drop pub, though that first needs its direct test/bench calls moved onto send.

Caveat: batch produce may be reserved for the planned 2-stage/batching pipeline (perf parity with Debezium). If that path is still on the roadmap, keep produce and close this. Otherwise it is speculative dead code, re-addable from git history when the batching work lands.

Ask: remove produce and its kafka_bench cases. Optionally tighten sendMessage visibility.