Remove deprecated function _update_create_fetch_task and its usage

Remove deprecated function _update_create_fetch_task and its usage

Usages:
1. update.module

/**
* Creates a new fetch task after loading the necessary include file.
*
* @param $project
*   Associative array of information about a project. See update_get_projects()
*   for the keys used.
*
* @see _update_create_fetch_task()
*/
function update_create_fetch_task($project) {
  module_load_include('inc', 'update', 'update.fetch');
  _update_create_fetch_task($project);
}

2. UpdateCoreTest.php

 /**
   * Tests that exactly one fetch task per project is created and not more.
   */
  function testFetchTasks() {
    $projecta = array(
      'name' => 'aaa_update_test',
    );
    $projectb = array(
      'name' => 'bbb_update_test',
    );
    $queue = \Drupal::queue('update_fetch_tasks');
    $this->assertEqual($queue->numberOfItems(), 0, 'Queue is empty');
    update_create_fetch_task($projecta);
    $this->assertEqual($queue->numberOfItems(), 1, 'Queue contains one item');
    update_create_fetch_task($projectb);
    $this->assertEqual($queue->numberOfItems(), 2, 'Queue contains two items');
    // Try to add project a again.
    update_create_fetch_task($projecta);
    $this->assertEqual($queue->numberOfItems(), 2, 'Queue still contains two items');

    // Clear storage and try again.
    update_storage_clear();
    drupal_static_reset('_update_create_fetch_task');
    update_create_fetch_task($projecta);
    $this->assertEqual($queue->numberOfItems(), 2, 'Queue contains two items');
  }

Function
update.fetch.inc

/**
 * Adds a task to the queue for fetching release history data for a project.
 *
 * We only create a new fetch task if there's no task already in the queue for
 * this particular project (based on 'update_fetch_task' key-value collection).
 *
 * @param $project
 *   Associative array of information about a project as created by
 *   update_get_projects(), including keys such as 'name' (short name), and the
 *   'info' array with data from a .info.yml file for the project.
 *
 * @see \Drupal\update\UpdateFetcher::createFetchTask()
 *
 * @deprecated in Drupal 8.x-dev, will be removed before Drupal 8.0.
 *   Use \Drupal::service('update.processor')->createFetchTask().
 */
function _update_create_fetch_task($project) {
  \Drupal::service('update.processor')->createFetchTask($project);
}