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/views/explore/posts/intro.html.erb

@@ -1,30 +0,0 @@
-<div id="c-explore-posts">
-  <div id="a-intro">
-    <header>
-      <h1><%= link_to Danbooru.config.app_name, posts_path %></h1>
-      <p class="byline">Find good anime art fast</p>
-
-      <%= form_tag(posts_path, :method => :get) do %>
-        <%= text_field_tag "tags", "", :size => 50, :data => { :autocomplete => "tag-query" } %>
-        <%= submit_tag "Search" %>
-      <% end %>
-    </header>
-
-    <p style="text-align: center;">Type in your favorite anime, manga, or character (last name first). Here are some popular examples:</p>
-
-    <% cache("intro-page", :expires_in => 1.hour) do %>
-      <% @presenter.each do |tag, post_set| %>
-        <div class="posts">
-          <h2><%= link_to tag, posts_path(:tags => tag) %></h2>
-          <%= post_set.presenter.post_previews_html(self, show_cropped: true) %>
-        </div>
-      <% end %>
-    <% end %>
-  </div>
-</div> 
-
-<%= render "static/footer" %>
-
-<% content_for(:page_title) do %>
-  <%= Danbooru.config.app_name %>
-<% end %>