chore: remove nullable controller extension

lib/src/controller/quill_controller_ext.dart

@@ -1,22 +0,0 @@
-import 'package:flutter/widgets.dart' show BuildContext;
-
-import 'provider.dart';
-import 'quill_controller.dart';
-
-// TODO: Remove this file later
-
-@Deprecated('No longer used internally and will be removed in future releases')
-extension QuillControllerNullableExt on QuillController? {
-  /// Return the controller from the widget tree using [context].
-  /// Throw if null.
-  @Deprecated(
-    'No longer used internally and will be removed in future releases',
-  )
-  QuillController notNull(BuildContext context) {
-    final controller = this;
-    if (controller != null) {
-      return controller;
-    }
-    return context.requireQuillController;
-  }
-}