posts: remove unused file path / url methods.

* Remove file_path_for, cropped_file_url (dead code)
* Remove complete_preview_file_url (preview_file_url now returns absolute links)
* Remove `file_name` (only used for Download link in sidebar)

app/presenters/post_presenter.rb

@@ -135,6 +135,10 @@ class PostPresenter < Presenter
     @post.humanized_essential_tag_string
   end
 
+  def filename_for_download
+    "#{humanized_essential_tag_string} - #{@post.md5}.#{@post.file_ext}"
+  end
+
   def categorized_tag_groups
     string = []