Remove dependency on BrowserThread::FILE from Sync code

ios/chrome/browser/sync/ios_chrome_sync_client.mm

Index: ios/chrome/browser/sync/ios_chrome_sync_client.mm
diff --git a/ios/chrome/browser/sync/ios_chrome_sync_client.mm b/ios/chrome/browser/sync/ios_chrome_sync_client.mm
index d066897389a38fff3855629d0161ffba49403250..29362a25fb5872dc701ce447d17dfddf34119ec1 100644
--- a/ios/chrome/browser/sync/ios_chrome_sync_client.mm
+++ b/ios/chrome/browser/sync/ios_chrome_sync_client.mm
@@ -382,10 +382,6 @@ IOSChromeSyncClient::CreateModelWorkerForGroup(syncer::ModelSafeGroup group) {
       return new syncer::SequencedModelWorker(
           web::WebThread::GetTaskRunnerForThread(web::WebThread::DB),
           syncer::GROUP_DB);
-    case syncer::GROUP_FILE:
-      return new syncer::SequencedModelWorker(
-          web::WebThread::GetTaskRunnerForThread(web::WebThread::FILE),
-          syncer::GROUP_FILE);
     case syncer::GROUP_UI:
       return new syncer::UIModelWorker(
           web::WebThread::GetTaskRunnerForThread(web::WebThread::UI));
@@ -405,6 +401,7 @@ IOSChromeSyncClient::CreateModelWorkerForGroup(syncer::ModelSafeGroup group) {
       return new browser_sync::PasswordModelWorker(password_store_);
     }
     default:
+      NOTREACHED();
       return nullptr;
   }
 }