remove no-longer needed LSI accessor

LSI has been moved to another class (Jekyll::RelatedPosts),
but this was left in Post.

lib/jekyll/post.rb

@@ -3,10 +3,6 @@ module Jekyll
     include Comparable
     include Convertible
 
-    class << self
-      attr_accessor :lsi
-    end
-
     # Valid post name regex.
     MATCHER = /^(.+\/)*(\d+-\d+-\d+)-(.*)(\.[^.]+)$/