Remove unused SwfAsset#images method

I think this used to be used in an API endpoint we've now deleted? I'm
just cleaning up call sites because I intend to refactor the `urls`
method and stuff, so I'm removing cruft that would complicate it!

I'm not certain-certain this is unused, but I did a global search for
`\bimages\b` in the codebase, and didn't find anything that looked like
a match to me!

app/models/swf_asset.rb

@@ -52,10 +52,6 @@ class SwfAsset < ApplicationRecord
       "#{image_version}"
   end
 
-  def images
-    IMAGE_SIZES.values.map { |size| {:size => size, :url => image_url(size)} }
-  end
-
   attr_accessor :item
 
   has_one :contribution, :as => :contributed, :inverse_of => :contributed