drop "ebooks" template from all book sidebars

We dropped this from the main book page in git#983, but the
template still appears in the individual section pages. The
same rationale applies, and we should drop it everywhere.

We can also drop the templates entirely (we may bring back
the links later, but we can resurrect them at that time).

Author: peff

app/views/books/_ebooks.html.erb

@@ -2,7 +2,6 @@
 <% @subsection = 'book' %>
 
 <% content_for :sidebar do %>
-  <%= render 'ebooks' %>
   <%= render 'translations' %>
   <%= render 'shared/related' %>
 <% end %>

app/views/books/section.html.erb

@@ -3,7 +3,6 @@
 <%- @page_title = "Git - Book" %>
 
 <% content_for :sidebar do %>
-  <%= render 'ebooks' %>
   <%= render 'translations' %>
 <% end %>
 

app/views/doc/_ebooks.html.erb

@@ -2,7 +2,6 @@
 <% @subsection = 'book' %>
 
 <% content_for :sidebar do %>
-  <%= render 'ebooks' %>
   <%= render 'translations' %>
   <%= render 'shared/related' %>
 <% end %>