removed useless element attr for underline

lib/src/packages/quill_markdown/markdown_to_delta.dart

@@ -83,7 +83,6 @@ class MarkdownToDelta extends Converter<String, Delta>
   final _elementToInlineAttr = <String, ElementToAttributeConvertor>{
     'em': (_) => [Attribute.italic],
     'u': (_) => [Attribute.underline],
-    'ins': (_) => [Attribute.underline],
     'strong': (_) => [Attribute.bold],
     'del': (_) => [Attribute.strikeThrough],
     'a': (element) => [LinkAttribute(element.attributes['href'])],