Revert "Issue #2154563 by chx: Migrate process plugin: flatten."

Oops. :P This is why I should not try and fix docs. :P

This reverts commit 9beae4fe.

core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/Flatten.php

-<?php
-
-/**
- * @file
- * Contains \Drupal\migrate\Plugin\migrate\process\Flatten.
- */
-
-namespace Drupal\migrate\Plugin\migrate\process;
-use Drupal\migrate\MigrateExecutable;
-use Drupal\migrate\ProcessPluginBase;
-use Drupal\migrate\Row;
-
-/**
- * This plugin flattens the current value.
- *
- * During some types of processing (e.g. user permission splitting), what was
- * once a single value gets transformed into multiple values. This plugin will
- * flatten them back down to single values again.
- *
- * @see https://drupal.org/node/2154215
- *
- * @MigrateProcessPlugin(
- *   id = "flatten",
- *   handle_multiples = TRUE
- * )
- */
-class Flatten extends ProcessPluginBase {
-
-  /**

core/modules/migrate/tests/Drupal/migrate/Tests/process/FlattenTest.php

-<?php
-/**
- * @file
- * Contains \Drupal\migrate\Tests\process\FlattenTest.
- */
-
-namespace Drupal\migrate\Tests\process;
-use Drupal\migrate\Plugin\migrate\process\Flatten;
-
-
-/**
- * Tests the flatten plugin.
- *
- * @group Drupal
- * @group migrate
- */
-class FlattenTest extends MigrateProcessTestCase {
-
-  /**
-   * {@inheritdoc}
-   */
-  public static function getInfo() {
-    return array(
-      'name' => 'Flatten process plugin',
-      'description' => 'Tests the flatten process plugin.',
-      'group' => 'Migrate',
-    );
-  }
-
-  /**
-   * Test that various array flatten operations work properly.
-   */
-  public function testFlatten() {
-    $plugin = new Flatten(array(), 'flatten', array());
-    $flattened = $plugin->transform(array(1, 2, array(3, 4, array(5)), array(), array(7, 8)), $this->migrateExecutable, $this->row, 'destinationproperty');
-    $this->assertSame($flattened, array(1, 2, 3, 4, 5, 7, 8));
-  }
-
-}