tags: eliminate Tag.category_for method.

Tag.category_for looked up a tag's category in the Redis cache. This was
only used in a few places (in related tags, and on the popular/missed
search pages). Get rid of this method so we can work towards getting rid
of caching tag categories in Redis.

app/logical/post_query_builder.rb

@@ -985,6 +985,11 @@ class PostQueryBuilder
     def is_wildcard_search?
       is_single_tag? && tags.first.wildcard
     end
+
+    def simple_tag
+      return nil if !is_simple_tag?
+      Tag.find_by_name(tags.first.name)
+    end
   end
 
   memoize :split_query, :normalized_query