9 – Create xml field without formatter

I am following Drupal official Custom field type guide to create a field which will store “XML” value, and the field will be the field of taxonomy.

This is same for defining a field type.

However, what I can not figure out is – how to create without field formatter. Cause, we want to keep the formatter hidden, and for internal storage purpose. The xml field will not be viewed to front-end.

 * Plugin implementation of the 'field_example_rgb' field type.
 * @FieldType(
 *   id = "field_example_rgb",
 *   label = @Translation("Example Color RGB"),
 *   module = "field_example",
 *   description = @Translation("Demonstrates a field composed of an RGB color."),
 *   default_widget = "field_example_text",

Above is the example from example module which shows use of field_types however I can not figure out is how to disable, or remove the default formatter.

When I write * default_formatter = {} This actually does not give any error, but the field can not be created.

How to create new field type and store xml value, so that it is not formatted, and it is not shown to front-end.