Remove LOGGER.exception() line

* Information logged in this line will be available to the calling code
  via the exception thrown.
* There are scenarios where due to concurrency issues,  there are two uncoordinated outputs happening at once. This may result in this line logging stack traces after log statements in the exception block of the calling code.

google/cloud/pubsub_v1/publisher/_batch/thread.py

@@ -290,7 +290,6 @@ def _commit(self) -> None:
                 # Failed to publish batch.
                 self._batch_done_callback(batch_transport_succeeded)
 
-            _LOGGER.exception("Failed to publish %s messages.", len(self._futures))
             return
 
         end = time.time()