explore/posts/intro: remove page.

This was an alternate frontpage that contained a list of previews of the
most popular tags. This page was never linked from anywhere and it was
unknown by most users.

app/logical/post_sets/intro.rb

@@ -1,15 +0,0 @@
-module PostSets
-  class Intro < PostSets::Post
-    def initialize(tags)
-      super(tags)
-    end
-
-    def posts
-      @posts ||= begin
-        temp = ::Post.tag_match("#{tag_string} favcount:>3").paginate(page, :search_count => nil, :limit => 5)
-        temp.each # hack to force rails to eager load
-        temp
-      end
-    end
-  end
-end