presenters: merge PostSetPresenter into PostSet.

Reduce indirection. PostSet is basically a collection of helper methods
for rendering the post index page. PostSetPresenter was a set of helper
methods for rendering the tag list on the post index page. These don't
need to be separated.

app/views/posts/index.html.erb

@@ -7,7 +7,7 @@
 
   <section id="tag-box">
     <h1>Tags</h1>
-    <%= @post_set.presenter.tag_list_html(current_query: params[:tags], show_extra_links: policy(Post).show_extra_links?) %>
+    <%= @post_set.tag_list_html(current_query: params[:tags], show_extra_links: policy(Post).show_extra_links?) %>
   </section>
 
   <%= render "posts/partials/index/options" %>