message_edit: Remove obsolete block for diff highlighting.

We disabled the original "colorized HTML edit-history" feature way
back in 2013 in c51056ff8e.

That original feature involved showing what had been edited inline in
message bodies, so one could easily see what had been changed.

That old feature has since been replaced with the "view edit history"
menu option, and we're unlikely to ever want the old feature back.
So, we can just remove its code.  There's a few supporting variables
that were created to help implement this; we can clean those up and
simplify the `update_message` code now that this feature is fully
removed.

zerver/lib/actions.py

@@ -3605,28 +3605,11 @@ def do_update_message(user_profile: UserProfile, message: Message, topic_name: O
         stream_id = message.recipient.type_id
         event['stream_name'] = Stream.objects.get(id=stream_id).name
 
-    # Set first_rendered_content to be the oldest version of the
-    # rendered content recorded; which is the current version if the
-    # content hasn't been edited before.  Note that because one could
-    # have edited just the topic_name, not every edit history event
-    # contains a prev_rendered_content element.
-    first_rendered_content = message.rendered_content
-    if message.edit_history is not None:
-        edit_history = ujson.loads(message.edit_history)
-        for old_edit_history_event in edit_history:
-            if 'prev_rendered_content' in old_edit_history_event:
-                first_rendered_content = old_edit_history_event['prev_rendered_content']
-
     ums = UserMessage.objects.filter(message=message.id)
 
     if content is not None:
         update_user_message_flags(message, ums)
 
-        # We are turning off diff highlighting everywhere until ticket #1532 is addressed.
-        if False:
-            # Don't highlight message edit diffs on prod
-            rendered_content = highlight_html_differences(first_rendered_content, rendered_content)
-
         # One could imagine checking realm.allow_edit_history here and
         # modifying the events based on that setting, but doing so
         # doesn't really make sense.  We need to send the edit event
@@ -3719,6 +3702,7 @@ def do_update_message(user_profile: UserProfile, message: Message, topic_name: O
     event['edit_timestamp'] = datetime_to_timestamp(message.last_edit_time)
     edit_history_event['timestamp'] = event['edit_timestamp']
     if message.edit_history is not None:
+        edit_history = ujson.loads(message.edit_history)
         edit_history.insert(0, edit_history_event)
     else:
         edit_history = [edit_history_event]