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

@@ -51,10 +51,6 @@ class SwfAsset < ApplicationRecord
     "https://impress-asset-images.openneo.net/#{image_dir}/#{size_key}.png?" +
       "#{image_version}"
   end
-  
-  def images
-    IMAGE_SIZES.values.map { |size| {:size => size, :url => image_url(size)} }
-  end
 
   attr_accessor :item