remove useless method

`images` method has been deleted in 2bc48561, `create_images_directory` does nothing.

railties/lib/rails/generators/rails/plugin/plugin_generator.rb

@@ -237,10 +237,6 @@ def create_javascript_files
         build(:javascripts) unless api?
       end
 
-      def create_images_directory
-        build(:images) unless api?
-      end
-
       def create_bin_files
         build(:bin)
       end