Remove gunk from post stream serializer mixin (#4176)

app/serializers/post_stream_serializer_mixin.rb

@@ -16,10 +16,8 @@ module PostStreamSerializerMixin
   def posts
     return @posts if @posts.present?
     @posts = []
-    highest_number_in_posts = 0
     if object.posts
-      object.posts.each_with_index do |p, idx|
-        highest_number_in_posts = p.post_number if p.post_number > highest_number_in_posts
+      object.posts.each do |p|
         ps = PostSerializer.new(p, scope: scope, root: false)
         ps.add_raw = true if @options[:include_raw]
         ps.topic_view = object