Remove monkey-patch in Mime::NullType

config/initializers/mime_types.rb

@@ -2,17 +2,3 @@
 
 # Add new mime types for use in respond_to blocks:
 # Mime::Type.register "text/richtext", :rtf
-
-# Extracted from https://github.com/rails/rails/blob/6-1-stable/actionpack/lib/action_dispatch/http/mime_type.rb#L344-L346
-# Mime::NullType has an odd behaviour in Rails 6 due to the check performed in
-# https://github.com/rails/rails/blob/6-0-stable/actionpack/lib/action_controller/metal/mime_responds.rb#L208-L210
-# where media_type and format are different even though both are Mime::NullType.
-# The to_s method fixes the issue but, unfortunately, it was added only for Rails 6.1+.
-# We may removed this patch once we upgrade Rails to 6.1.
-module Mime
-  class NullType
-    def to_s
-      ''
-    end
-  end
-end