title: Boto3 - delete all items with a specific partition key
body: <p>I have a table with a partition key and sort key.</p>
  <p>I would like to delete all items that belong to a partition key. But when I call delete_item with only the partition key it doesn't work:</p>
  <pre><code>table.delete_item(Key={&quot;user_id&quot;: &quot;xx&quot;})
  </code></pre>
  <p>I get this error:</p>
  <pre><code>botocore.exceptions.ClientError: An error occurred (ValidationException) when calling the DeleteItem operation: The provided key element does not match the schema
  </code></pre>