Remove first_line_in_markdown is_todo kwarg
What does this MR do and why?
Remove MarkupHelper.first_line_in_markdown
is_todo
keyword argument
All references to this keyword argument were removed in Remove two_line_mention_enabled feature flag (!108295 - merged) so it is no longer required.
This was picked up while reviewing Exposed note body first line on GraphQL (!148829 - merged)