batch_mode-ectomy to fix recursive bucket_list

batch_mode was introduced to try to let deletes happen using the
BATCH_DELETE operator.  Unfortunately, that introduced some side
effects, specifically now only the first 1000 files in a bucket were
retrieved during operation del --recursive.  Furthermore, since
6f91f384, we no longer need the batch_mode logic as we are dealing
with slices of the remote_list.

This patch specifically removes the conditional as follows:

-            truncated = _list_truncated(response["data"]) and not batch_mode
+            truncated = _list_truncated(response["data"])

returning it to its pre-batch_mode glory, such that the entire list is
recursively obtained properly.

Furthermore, delete all references to batch_mode, as it is no longer
used anywhere after the above change.