Remove image_optim config loading override

image_optim_rails loads the config files by default, and has a
built-in mechanism for per-environment configs. Forcing the config
loading in this way is unnecessary.

config/application.rb

@@ -43,8 +43,5 @@ module OpenStreetMap
       config.logstasher.logger_path = LOGSTASH_PATH
       config.logstasher.log_controller_parameters = true
     end
-
-    # Configure image optimisation
-    config.assets.image_optim = YAML.load_file(Rails.root.join("config", "image_optim.yml"))
   end
 end