Remove deprecated `ConnectionError` (#1816)
Remove deprecated `ConnectionError` (#1816)

This has been deprecated for a bit in favor of `KafkaConnectionError`
because it conflicts with Python's built-in `ConnectionError`.

Time to remove it as part of cleaning up our old deprecated code.
mumrah/kafka-python,mumrah/kafka-python,scrapinghub/kafka-python,ohmu/kafka-python,dpkp/kafka-python,ohmu/kafka-python,scrapinghub/kafka-python,dpkp/kafka-python,DataDog/kafka-python