Remove #tree and #attributes properties from LinkWidget::formElement on element of #type 'value'

Problem/Motivation

According to the Form API Reference an element of #type value can only use the properties #type and #value.

LinkWidget::formElement however defines #tree and #attributes. Especially the CSS class does not make any sense, since according to the FAPI reference a value is:

A form value that is internal to the form and never displayed to the screen.

    // Exposing the attributes array in the widget is left for alternate and more
    // advanced field widgets.
    $element['attributes'] = array(
      '#type' => 'value',
      '#tree' => TRUE,
      '#value' => !empty($items[$delta]->options['attributes']) ? $items[$delta]->options['attributes'] : array(),
      '#attributes' => array('class' => array('link-field-widget-attributes')),
    );

According to the patch #4 the attributes are need to migrate from Drupal 6. Then, we need to check how these attributes are used in D6 in order to check if they can be migrated as configuration data we already have in D8.

Proposed resolution

Remaining tasks

Check how attributes are used in D6 in order to check if they can be migrated as configuration data we already have in D8.

User interface changes

API changes

Data model changes