FIX: Consistently handle category param

See https://meta.discourse.org/t/api-post-to-posts-json-inconsistent-between-users/118571
for more info.

This commit removes a 5 year old temporary fix that is no longer needed.

bc1824a6ed (diff-d8c648926664f849aec050757bfcb6f9R72)

The web interface uses category_id when creating a topic so I think we
should unify on category_id when using the api.

lib/topic_creator.rb

@@ -153,13 +153,8 @@ class TopicCreator
         return Category.find(SiteSetting.shared_drafts_category)
       end
 
-      # Temporary fix to allow older clients to create topics.
-      # When all clients are updated the category variable should
-      # be set directly to the contents of the if statement.
       if (@opts[:category].is_a? Integer) || (@opts[:category] =~ /^\d+$/)
         Category.find_by(id: @opts[:category])
-      else
-        Category.find_by(name_lower: @opts[:category].try(:downcase))
       end
     end
   end