Remove unused Item.per_page attribute

I feel like this was part of `will_paginate` back before the Rails
community had itself figured out about what belongs in a model?

But yeah, a default per-page value for search results does not belong
here. And I don't think anything references it anymore, because we pass
`per_page` to the `paginate` call in `ItemsController` explicitly! So,
goodbye!

app/models/item.rb

@@ -18,9 +18,6 @@ class Item < ApplicationRecord
   SPECIAL_COLOR_DESCRIPTION_REGEX =
     /This item is only wearable by [a-zA-Z]+ painted ([a-zA-Z]+)\.|WARNING: This [a-zA-Z]+ can be worn by ([a-zA-Z]+) [a-zA-Z]+ ONLY!|If your Neopet is not painted ([a-zA-Z]+), it will not be able to wear this item\./
 
-  cattr_reader :per_page
-  @@per_page = 30
-
   scope :newest, -> {
     order(arel_table[:created_at].desc) if arel_table[:created_at]
   }