Remove consumer assignment for backups

When instantiating the consumer we already a subscribe to the topic that
is to be backed up (ie. __schemas). Using assign on top of that causes
problems - once the assignment actually takes effect, the consumer
starts to consume from the beginning, ultimately resulting in an
`InconsistentOffset` error.

The default subscribe is enough at the moment, as we do not support
backing up a miltui-partition topic.

karapace/backup/api.py

@@ -530,7 +530,6 @@ def create_backup(
         with _consumer(config, topic_name) as consumer:
             (partition,) = consumer.partitions_for_topic(topic_name)
             topic_partition = TopicPartition(topic_name, partition)
-            consumer.assign([topic_partition])
 
             try:
                 data_file = _write_partition(