fixup! post: review Post API

setting a title was a mistake, because it will just be HTML content. No
title at all is better.

bt5/erp5_post/SkinTemplateItem/portal_skins/erp5_post/PostModule_createHTMLPostFromText.py

 return context.PostModule_createHTMLPost(
-  title=data.splitlines()[0][:30] if data else None,
   source_reference=source_reference,
   data=data,
   follow_up=follow_up,