AjaxRequestCheck.php

<?php

use Drupal\Core\ContentNegotiation;
use Symfony\Component\HttpFoundation\Request;

class AjaxRequestCheck {
  public function __construct(ContentNegotiation $negotiation, Request $request) {
    $this->negotiation = $negotiation;
    $this->request = $request;
  }

  public function isAjax() {
    return bool $this->negotiation->getContentType($this->request) == 'ajax';
  }
}

CoreBundle.php

<?php

// In Drupal/Core/CoreBundle.php

$container->register('ajax_request_check', 'Drupal\Core\AjaxRequestCheck')
  ->setArgument(new Reference('content_negotiation'))
  ->setArgument(new Reference('request'));

Drupal\some\Routing\MyForm.php

<?php

use Drupal\Core\Form\FormInterface;
use Drupal\Core\ControllerInterface

class MyForm implements FormInterface, ControllerInterface {
  public function __construct(AjaxRequestCheck $check) {
    $this->check = $check;
  }

  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('ajax_request_check')
    );
  }

  public function buildForm(array $form, array &$form_state) {
    $form['delete'] = array(
      '#type' => 'submit',
      '#value' => 'Delete',
    );
    if ($this->check->isAjax()) {
      $form['delete']['#ajax']['callback'] = 'function_to_close_modal';
    }
  }
}

Drupal\some\SomeBundle.php

<?php

namespace Drupal\some;

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\HttpKernel\Bundle\Bundle;

class SomekBundle extends Bundle {

  /**
   * Overrides \Symfony\Component\HttpKernel\Bundle\Bundle::build().
   */
  public function build(ContainerBuilder $container) {
    $container->register('some.form', 'Drupal\some\Routing\MyForm')
      ->setArgument(new Reference('ajax_request_check'));
  }

}

some.routing.yml

some.form:
  pattern: '/admin/config/some_form'
  defaults:
    _form: 'Drupal\some\Routing\MyForm'
  requirements:
    _permission: 'administer some form'