related_tag_calculator.rb: fix memcache lookup in inner loop.

Remove the category constraint option from RelatedTagCalculator.calculate_from_posts.
It slows things down and isn't used.

This method is used to calculate the related tags sidebar during
searches for single metatags. Using Tag.category_for in the inner loop
caused a memcache call on every iteration. At 100 posts per page and
20-30 tags per post, this led to up to 2000-3000 total memcache calls,
which significantly slowed pageloads.

app/presenters/post_set_presenters/post.rb

@@ -1,11 +1,14 @@
 module PostSetPresenters
   class Post < Base
-    attr_accessor :post_set, :tag_set_presenter
+    attr_accessor :post_set
     delegate :posts, :to => :post_set
 
     def initialize(post_set)
       @post_set = post_set
-      @tag_set_presenter = TagSetPresenter.new(related_tags)
+    end
+
+    def tag_set_presenter
+      @tag_set_presenter ||= TagSetPresenter.new(related_tags)
     end
 
     def related_tags
@@ -51,7 +54,7 @@ module PostSetPresenters
     end
 
     def calculate_related_tags_from_post_set
-      RelatedTagCalculator.calculate_from_post_set_to_array(post_set).map(&:first)
+      RelatedTagCalculator.calculate_from_posts_to_array(post_set.posts).map(&:first)
     end
 
     def saved_search_labels