NEW Removed migrate_legacy_file support

For the vast majority of sites, you really don't want to run your file migration as part of dev build.
The step is involved enough to warrant it's own task.
I don't think this is an API change, since the setting won't have affect
for anyone who has already enabled it - they would've already done the one-off migration.

src/File.php

@@ -255,14 +255,6 @@ class File extends DataObject implements AssetContainer, Thumbnail, CMSPreviewab
      */
     private static $apply_restrictions_to_admin = true;
 
-    /**
-     * If enabled, legacy file dataobjects will be automatically imported into the APL
-     *
-     * @config
-     * @var bool
-     */
-    private static $migrate_legacy_file = false;
-
     /**
      * @config
      * @var boolean
@@ -1206,21 +1198,6 @@ public function getTag()
         return (string)$this->renderWith($template);
     }
 
-    public function requireDefaultRecords()
-    {
-        parent::requireDefaultRecords();
-
-        // Check if old file records should be migrated
-        if (!$this->config()->get('migrate_legacy_file')) {
-            return;
-        }
-
-        $migrated = FileMigrationHelper::singleton()->run();
-        if ($migrated) {
-            DB::alteration_message("{$migrated} File DataObjects upgraded", "changed");
-        }
-    }
-
     /**
      * Get the back-link tracking objects that link to this file via HTML fields
      *

tests/php/Dev/Tasks/FileMigrationHelperTest.php

@@ -43,8 +43,6 @@ protected function getBasePath()
 
     public function setUp()
     {
-        Config::nest(); // additional nesting here necessary
-        Config::modify()->merge(File::class, 'migrate_legacy_file', false);
         parent::setUp();
 
         // Set backend root to /FileMigrationHelperTest/assets
@@ -77,7 +75,6 @@ public function tearDown()
         TestAssetStore::reset();
         Filesystem::removeFolder($this->getBasePath());
         parent::tearDown();
-        Config::unnest();
     }
 
     /**

tests/php/LegacyThumbnailMigrationHelperTest.php

@@ -43,8 +43,6 @@ protected function getBasePath()
 
     public function setUp()
     {
-        Config::nest(); // additional nesting here necessary
-        Config::modify()->merge(File::class, 'migrate_legacy_file', false);
         parent::setUp();
 
         // Set backend root to /LegacyThumbnailMigrationHelperTest/assets
@@ -68,7 +66,6 @@ public function tearDown()
         TestAssetStore::reset();
         Filesystem::removeFolder($this->getBasePath());
         parent::tearDown();
-        Config::unnest();
     }
 
     public function testMigratesThumbnailsForProtectedFiles()