erp5_discussion: no need to sort post to generate the full text content

bt5/erp5_discussion/SkinTemplateItem/portal_skins/erp5_discussion/DiscussionThread_getChildDiscussionPostRawText.py

 child_raw_text = ""
 
-child_brain_list = context.DiscussionThread_getContextPostList(
-  sort_on=['creation_date', 'descending'],
-  )
+child_brain_list = context.DiscussionThread_getContextPostList()
 for child in child_brain_list:
   child_raw_text += child.asRawText()
 #XX If word is last, it is not displayed