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.

test/unit/related_tag_calculator_test.rb

@@ -18,11 +18,11 @@ class RelatedTagCalculatorTest < ActiveSupport::TestCase
         FactoryGirl.create(:post, :tag_string => "aaa bbb ccc ddd")
         FactoryGirl.create(:post, :tag_string => "aaa bbb ccc")
         FactoryGirl.create(:post, :tag_string => "aaa bbb")
-        @post_set = PostSets::Post.new("aaa")
+        @posts = Post.tag_match("aaa")
       end
 
       should "calculate the related tags" do
-        assert_equal({"aaa"=>3, "bbb"=>3, "ccc"=>2, "ddd"=>1}, RelatedTagCalculator.calculate_from_post_set(@post_set))
+        assert_equal({"aaa"=>3, "bbb"=>3, "ccc"=>2, "ddd"=>1}, RelatedTagCalculator.calculate_from_posts(@posts))
       end
     end