Remove deprecated ConnectionError

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.

Author: jeffwidman

kafka/errors.py

@@ -462,10 +462,6 @@ class KafkaConnectionError(KafkaError):
     invalid_metadata = True
 
 
-class ConnectionError(KafkaConnectionError):
-    """Deprecated"""
-
-
 class BufferUnderflowError(KafkaError):
     pass