wiki pages: remove category_name field from API.

Remove the `category_name` field from the `/wiki_page.json` API. This
field was originally added only because it was needed by our autocomplete
Javascript. It was also misnamed, it wasn't the tag's category name, it
was the category's ID.

Users should use `https://danbooru.donmai.us/wiki_pages.json?only=title,tag`
instead if they need this.

This triggered a N+1 query pattern when dumping wiki pages to BigQuery,
which made dumping wiki pages very slow. It also meant this field was
included in the database dump, even though it wasn't a real database
column.

app/models/wiki_page.rb

@@ -162,10 +162,6 @@ class WikiPage < ApplicationRecord
     name.to_s.unicode_normalize(:nfkc).normalize_whitespace.gsub(/[[:space:]]+/, "_").squeeze("_").gsub(/\A_|_\z/, "")
   end
 
-  def category_name
-    tag&.category
-  end
-
   def pretty_title
     title.tr("_", " ")
   end

app/policies/wiki_page_policy.rb

@@ -14,12 +14,4 @@ class WikiPagePolicy < ApplicationPolicy
   def permitted_attributes
     [:title, :body, :other_names, :other_names_string, :is_deleted, (:is_locked if can_edit_locked?)].compact
   end
-
-  def api_attributes
-    super + [:category_name]
-  end
-
-  def html_data_attributes
-    super + [:category_name]
-  end
 end

app/views/wiki_page_versions/diff.html.erb

@@ -4,7 +4,7 @@
 <%= render "wiki_pages/sidebar" %>
 
 <% content_for(:content) do %>
-  <h1>Wiki Page Version Comparison:  <%= link_to @thispage.title, wiki_page_versions_path(search: { wiki_page_id: @thispage.wiki_page_id }, type: params[:type], anchor: "wiki-page-version-#{@thispage.id}"), class: "tag-type-#{@thispage.wiki_page.category_name}" %></h1>
+  <h1>Wiki Page Version Comparison: <%= link_to @thispage.title, wiki_page_versions_path(search: { wiki_page_id: @thispage.wiki_page_id }, type: params[:type], anchor: "wiki-page-version-#{@thispage.id}"), class: "tag-type-#{@thispage.wiki_page.tag.category}" %></h1>
 
   <% if params[:type].present? %>
     <%= render "versions/types" %>