Remove Danbooru Winter Sale.

config/danbooru_default_config.rb

@@ -528,12 +528,6 @@ module Danbooru
     def redis_url
       "redis://localhost:6379"
     end
-
-    def is_promotion?
-      Time.use_zone("UTC") do
-        Time.zone.now < Time.zone.parse("2021-01-01")
-      end
-    end
   end
 
   EnvironmentConfiguration = Struct.new(:config) do