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.

test/functional/explore/posts_controller_test.rb

@@ -30,13 +30,6 @@ module Explore
           assert_response :success
         end
       end
-
-      context "#intro" do
-        should "render" do
-          get intro_explore_posts_path
-          assert_response :success
-        end
-      end
     end
   end
 end