Remove deprecated Consumer API poll(long timeout)

Per KIP-266, the Consumer.poll(long timeout) method was deprecated back in 2.0.0.

In 3.7, there are two implementations, each with different behavior:

• The LegacyKafkaConsumer implementation will continue to work but will log a warning about its removal
• The AsyncKafkaConsumer implementation will throw an error.

In 4.0, the `poll` method that takes a single `long` timeout will be removed altogether.