template_preprocess_field_multiple_value_form() remove nested render arrays

Problem/Motivation

Follow-up to #2584837: Double translation in template_preprocess_field_multiple_value_form()

Remove nested 'title' render array because that's two render arrays that we don't need.

    $header = array(
      array(
        'data' => array(
          '#prefix' => '<h4 class="label">',
          'title' => array(
            '#markup' => $element['#title'],
          ),
          '#suffix' => '</h4>',
        ),
        'colspan' => 2,
        'class' => array('field-label'),
      ),

Proposed resolution

Bring it down to one render array.

Remaining tasks

Ensure this still works through manual testing.