config: remove enable_image_cropping option.

There's no need to disable this option. Supporting it adds complexity
and disabling it is untested.

config/danbooru_default_config.rb

@@ -502,10 +502,6 @@ module Danbooru
     def recaptcha_secret_key
     end
 
-    def enable_image_cropping
-      true
-    end
-
     # Akismet API key. Used for Dmail spam detection. http://akismet.com/signup/
     def rakismet_key
     end